1. How to use an oscilloscope to test and analyze the reliability of a designed product?
Answer: Oscilloscopes have long been one of the most effective tools for testing electronic circuits. By observing the voltage and current waveforms at the key nodes of the circuit, you can visually check whether the circuit is working properly and verify whether the design is appropriate. This is extremely helpful for improving reliability. Of course, the correct analysis and judgment of the waveform depends on the engineer's own experience.
2. What are the main factors that determine the price of oscilloscope probes?
Answer: There are many types of oscilloscope probes with different performances, such as high voltage, differential, active high-speed probes, etc. The price ranges from a few hundred yuan to nearly ten thousand US dollars. The main determinants of price are of course bandwidth and function. The probe is the part of the oscilloscope that contacts the circuit. A good probe can provide the fidelity required for testing. To achieve this, even if it is a passive probe, there must be a lot of passive device compensation circuits (RC networks) inside.
3. How long is the service life of a general oscilloscope probe? Does the probe need to be calibrated regularly?
Answer: It is difficult to say the life of an oscilloscope probe. It depends on the environment and method of use.
The standard does not have clear measurement regulations for probes, but for passive probes, at least when replacing probes or exchanging channels, probe compensation adjustment must be performed. All active probes should be preheated for at least 20 minutes before use, and some active probes and current probes need zero drift adjustment.
4. What is the real-time sampling rate of an oscilloscope?
Answer: The real-time sampling rate refers to the reciprocal of the sampling interval of an oscilloscope's one acquisition (one trigger). It is understood that the highest level in the industry is the simultaneous use of four channels.
5. What is the equivalent time sampling of an oscilloscope?
Answer: Equivalent time sampling means that the oscilloscope pieces together the waveforms collected by multiple acquisitions (multiple triggers) into one waveform. The sampling rate may be very slow each time, and there is a certain offset between the two acquisition trigger points. The reciprocal of the minimum sampling interval between the two points formed in the end is called the equivalent sampling rate. Its index can reach a very high level, such as 1ps.
6. What is power factor? How to measure it?
Answer: Power factor: In a DC circuit, voltage multiplied by current is active power. But in an AC circuit, voltage multiplied by current is apparent power, and the part of power that can do work (i.e., active power) will be less than the apparent power. The ratio of active power to apparent power is called power factor, which is expressed as COSΦ. In fact, the simplest way to measure it is to measure the phase difference between voltage and current, and the result is power factor.
7. How to express and test power density?
Answer: Power density is the power per unit volume, and W/in3 is generally used in power supplies.
8. Is there any way to use an oscilloscope to measure the working conditions of a high-frequency transformer or inductor core?
Answer: There is a function in the power test solution launched by TEK - BH curve analysis, which can reflect the working state of the core, measure the dynamic inductance value, and obtain the core loss.
9. There are many types of noise in switching power supplies, such as cross interference caused by unreasonable wiring, inductor leakage, diode reverse spikes, etc. How to identify noise with an oscilloscope?
Answer: TEK's TDS5000 oscilloscope has frequency domain analysis. By analyzing the frequency band of noise, the type of noise can be analyzed, so that the corresponding processing method can be used. Oscilloscopes can only provide data analysis and band display.
10. How can an oscilloscope be used to test the radiation of the switching power supply?
Answer: There is radiation interference in the switching power supply. The general practice is to try to find the interference source and then shield it. The oscilloscope can use the Fourier transform function to analyze its frequency component composition, and determine the type of interference based on the frequency range.
11. In the design process of flyback power supply, the conversion efficiency of the transformer is often reduced due to the large leakage inductance of the transformer. When winding, the method of winding the secondary in the middle of the primary is still not ideal. Are there any skills in winding the transformer?
Answer: Wrap the high-power output winding inside, as close to the primary as possible, to strengthen the coupling.
12. Is there an oscilloscope that can analyze switching loss?
Answer: Tektronix's power supply test system, namely the TDS5000 series digital phosphor oscilloscope plus the TDSPWR2 power analysis software, can easily analyze the switching loss and the power loss per cycle, even including RDS ON.
13. Can the oscilloscope perform Fourier decomposition?
Answer: Most modern digital oscilloscopes have FFT functions, and the above system can even pre-test current harmonics according to the EN61000-3-2 standard.
14. Can the oscilloscope perform filtering? For example, low-pass filtering of PWM waves?
Answer: TDS5000 can perform 20MHz and 150MHz low-pass filtering, and can also perform a digital low-pass filtering called high-resolution acquisition. In this mode, the vertical resolution of the sampling point can be increased from 8 bits to 12 bits. The above system can output a sine wave waveform similar to that of a signal such as PWM according to the trend of pulse width change.
15. When using a digital oscilloscope, what are the principles for setting the B trigger and trigger level and the measured signal?
Answer: Tektronix's oscilloscope supports A, B trigger function. Simply put, it can trigger dual event sequence. When AB seq is selected, the A event is used as the main trigger, and the B event is used to capture complex waveforms. The trigger method is the A event arm trigger system, which triggers at the B event when the defined B event occurs. For detailed trigger instructions, please refer to the oscilloscope manual.
16. How to use TDS3052B to measure the maximum value of the modulated wave with a carrier frequency
of tens of K and a modulating wave frequency of the power supply frequency? Answer: The power frequency input may be a low frequency of 50Hz/60Hz, and the carrier frequency is tens of K. A power frequency cycle is about 20ms. If the oscilloscope needs to observe a 20ms signal, the duration acquisition window of the oscilloscope must be at least 2ms/div ×10 grids. At the same time, the sampling rate of the oscilloscope is determined based on the carrier signal of tens of K. Finally, the required acquisition memory length can be estimated to determine whether it can meet the test requirements.
17. Using a nominal 100MHz DSO oscilloscope to measure a high-frequency switch with an amplitude of 400V and f=50M, how does the oscilloscope depict its waveform and rise time?
Answer: ① The bandwidth of the oscilloscope is defined as the bandwidth of the sine wave amplitude attenuation -3dB point.
② The description of the waveform and rise time in the digital oscilloscope is obtained by obtaining waveform data through real-time sampling circuits and high-speed A/D converters, and then interpolation operations are performed.
③ In Tektronix's oscilloscope, there is a real-time processing circuit to complete the so-called sine interpolation function, which is completed in the signal acquisition circuit part. Of course, many oscilloscopes also complete mathematical operations through the main processor of the oscilloscope, which will take more time.
④ For the signal you measured, I am afraid that it is impossible to use a 100MHz oscilloscope. For a 50MHz square wave, in theory, an oscilloscope of more than 450MHz should be used to accurately reproduce the most important 9th harmonics in the signal, so as to ensure that the waveform is not distorted. What's more, you may also have to consider the signal rise time. In theory, the rise time of the oscilloscope should be more than 5 times faster than the signal.
⑤ The same is true for probes. Since ordinary probes will produce high-frequency distortion when measuring high voltages, you should use special differential probes or high-voltage probes such as Tektronix's P5205 and P5100 for measurement.
18. How to use digital oscilloscopes in analog circuits, such as measuring small signals of audio amplifiers, noise in power supplies, etc.?
Answer: The following issues need to be noted:
① The grounding of the oscilloscope. The reference ground wires of the oscilloscope housing and probes are connected to the ground wire, so good grounding is the primary condition for measuring interference.
② The interference introduced by the reference ground wire of the oscilloscope. Since ordinary probes usually have a ground wire, they will form an interference path similar to a loop antenna with the point to be measured, introducing relatively large interference. Therefore, to minimize this interference, you can remove the probe cap and do not use the ground wire led out from the probe, but directly use the probe tip and the ground inside the probe to contact the point to be measured for measurement.
③ Use differential measurement methods to eliminate common-mode noise. Tektronix provides a series of differential probes, such as the ADA400A specifically for small signals, which can measure hundreds of microvolts, and the P7350 for high-speed signal measurement provides a bandwidth of up to 5GHz.
④ Many Tektronix oscilloscopes provide a high-resolution acquisition (Hi-Res) signal capture mode, which can filter random noise superimposed on the signal.
19. When measuring the conducted disturbance of the signal line off the board, two large noise signals were found at two specific frequency points (one is 659K and the other is 1.977K). Preliminary analysis shows that it is caused by the switching power supply chip on the board. How to use an oscilloscope to measure such noise signals?
Answer: There are several factors to consider when testing noise signals with an oscilloscope:
① The amplitude of the measured signal, whether it is a small signal, the oscilloscope can test uA-level signals with the probe
② The frequency of the measured signal.
③ Improper connection of the probe will generate noise and affect the test results.
20. When using a Tektronix oscilloscope, how do you understand the Holdoff parameter?
Answer: Holdoff (trigger holdoff) means temporarily closing the trigger circuit of the oscilloscope for a period of time (i.e., holdoff time). During this period of time, the oscilloscope will not trigger even if there is a signal waveform point that meets the trigger condition. In digital oscilloscopes, it is also expressed as a percentage, which means the percentage of the entire record length or the entire screen.
The function of the trigger part of the oscilloscope is to display the waveform stably, and trigger holdoff is also a function set to display the waveform stably. It is specially designed for large-cycle repetitions where there are many non-repetitive waveform points that meet the trigger conditions. For example, as shown in the figure, the red points in the figure can all meet the trigger conditions. If the holdoff function is not used, the trigger point will not be fixed, resulting in unstable display. After using trigger holdoff, the trigger is triggered at the same point each time, so the display can be stable.
In addition, trigger holdoff is also required for amplitude modulation signals. For details, please refer to the Tektronix article "Oscilloscope XYZ".
21. Regarding holdoff, what is the difference between triggering and non-triggering, and how the oscilloscope processes the acquired signal?
Answer: For digital oscilloscopes, regardless of whether they are triggered or not, the oscilloscope is actually constantly acquiring waveforms, but only stable triggers can produce stable displays. This situation may also occur when the oscilloscope trigger circuit mode is in "automatic" mode, that is, the waveform is displayed regardless of whether the trigger conditions are met. If the "normal" mode is used, the waveform will not be displayed if the trigger conditions are not met.
22. Regarding holdoff, if the horizontal time resolution remains unchanged, does the larger the percentage setting (the corresponding signal display gradually stabilizes) mean that the signal cycle is longer?
A: Yes, the larger the percentage, the longer the holdoff time.
23. How to use an oscilloscope to measure differential signals?
A: The best way is to use a differential probe. The measured signal is the most real and objective. If there is no differential probe, you can use two differential probes to connect to the two channels of the oscilloscope (such as Ch1, Ch2), and then use mathematical operations to get the waveform of ch1-ch2 and analyze it. At this time, try to keep the two probes exactly the same, and the Vertical scale (how many volts per grid) of the two channels of the oscilloscope the same, otherwise, the error will be large.
24. How to use an oscilloscope to measure the differential signal on the USB bus?
A: There are two situations for testing USB signals:
The first is to comply with the physical layer test specifications of the USB1.1/2.0 bus defined by the USB organization. Only after passing the USB consistency test can the USB logo be marked. The USB physical layer consistency test is divided into many test items, which mainly examine the signal quality of the USB signal, such as
Signal Quality Test
Droop & Drop Test
Inrush Current Test
HS Specific Tests
Chirp Test
Monotonic Test
Receiver sensitivity Test
Impedance Test (TDR) and so on.
The second case is to observe only the signal on the USB bus. You can choose a suitable differential probe to connect to D+, D-, and directly observe the USB signal. The USB2.0 signal speed is relatively fast, with a rise time of several hundred picoseconds. In order to ensure the signal packet truth test, it is necessary to select an oscilloscope greater than 2GHz and a differential probe for testing.
25. High-speed signal characteristics on PCB board: XAUI interface 3.125GBd serial differential signal: 60ps, what bandwidth of oscilloscope is required for accurate measurement? What is the measurement error?
Answer: The 3.125GBd serial differential signal of the XAUI interface sounds a bit like an InfiniBand signal. It is collected by sinusoidal interpolation or similar equivalent sampling. However, due to factors such as bandwidth and trigger jitter, when measuring the rise time in the range of 100ps to 130ps, a 7GHz differential probe can ensure an error of <3%. For rise time measurement of < 80ps, the error will be greater than 10%. Although this is the best solution in a real-time oscilloscope, the most accurate solution for rise time measurement is Agilent's network analyzer (with physical layer analysis software) because its bandwidth can reach up to 50GHz.
26. For designs with high requirements for clock phase noise parameters, what key issues need to be considered to reduce phase noise?
Answer: There are many indicators for measuring performance in ADC and DAC devices: such as bit number, conversion speed, DC accuracy, switching performance, dynamic performance (SNR, SINAD, IMD), etc.
27. For designs with high requirements for clock phase noise parameters, how to measure phase noise?
A: From the perspective of the oscilloscope, the amplitude, time, converted signal quality, conversion speed, clock and data setup/hold time and other parameters of the analog and digital signals of the ADC and DAC can be tested. The advanced calculation function (spectrum analysis function) in the TDS oscilloscope can also be used to qualitatively measure parameters such as SNR and SINAD.
28. Since it may be necessary to introduce an external clock, there is a problem of choosing one from two. What solution can be used to minimize the deterioration of phase noise?
A: First, we need to analyze the source of jitter. Oscilloscope is a good tool for jitter analysis. Currently, we can use TDS5000B/6000B/7000B series oscilloscopes with jitter analysis software to conduct a thorough jitter analysis, such as determining jitter (Dj), random jitter (Rj), and separating Rj from Dj. Finally, we can eliminate jitter by analyzing the cause of jitter.
29. When viewing waveforms on an oscilloscope, what is the difference between using external triggering and self-triggering?
A: The usual trigger of an oscilloscope is edge triggering, which has two triggering conditions, trigger level and trigger edge; that is, when the rising edge (or falling edge) of the signal reaches a certain level (trigger level), the oscilloscope triggers. The oscilloscope will only use external triggering when there is a problem with the signal self-triggering. There is no better problem. This problem may usually be that the signal is relatively complex, there are many points that meet the trigger conditions, and it is impossible to trigger at the same position every time to obtain a stable display. At this time, an external trigger is needed. Take the following example:
Observe the signal above. Since each point ABCD will be triggered, the waveform displayed by the oscilloscope will not be stable. At this time, the following signal can be used as the trigger signal, and the oscilloscope will be able to display all cycles.
30. The bandwidth of TDS3032B is 300MHz, the sampling frequency is 2.5G/s, and the sampling frequency is 8 times the bandwidth. What is the fixed relationship between bandwidth and sampling frequency? We also have an oscilloscope from another manufacturer with a bandwidth of 100MHz and a sampling frequency of only 200MHz. Why is the bandwidth sampling frequency ratio of the two oscilloscopes so different?
Answer: Bandwidth is the most important indicator of an oscilloscope, because there is an ADC in a digital oscilloscope, and its sampling rate theoretically needs to meet the Nyquist sampling theorem, that is, each cycle of the highest frequency signal of the measured signal theoretically needs to sample at least 2 points, otherwise it will cause aliasing. But in reality, it also depends on many other factors, such as the waveform reconstruction algorithm. Tektronix oscilloscopes use advanced waveform reconstruction algorithms, and only 2.5 points are needed for each cycle of the measured signal to reconstruct the waveform. Some oscilloscopes use linear interpolation algorithms, which may require 10 points. Generally, a sampling rate of 4-5 times the bandwidth can accurately reproduce the waveform.
Tektronix's TDS3000B series is a "real-time sampling" oscilloscope, that is, its single-shot bandwidth (the ability to capture a single signal) = repetitive bandwidth. The single-shot bandwidth of the other oscilloscope you mentioned is obviously less than 100MHz. You can take a look at its indicators.
31. How to understand the bandwidth in the oscilloscope indicators?
Answer: Bandwidth is a basic indicator of an oscilloscope. It is the same as the definition of amplifier bandwidth. It is the so-called -3dB point, that is, the frequency point when the amplitude is attenuated to 70.7% of the actual amplitude when a sine wave is added to the input of the oscilloscope is called bandwidth. In other words, using an oscilloscope with a bandwidth of 100MHz to measure a 1V, 100MHz sine wave, the amplitude obtained is only 0.707V. This is only the case of a sine wave. Therefore, when we choose an oscilloscope, in order to achieve a certain measurement accuracy, we should choose a bandwidth that is 5 times the highest frequency of the signal.
32. How to obtain the total bandwidth of the measurement system?
Answer: The total bandwidth of the measurement system = 0.35/rise time (oscilloscopes below 1GHz).
33. Under the condition of a certain bandwidth, does it make little sense to have a too high sampling frequency?
Answer: Bandwidth is the basic condition to limit the capture of the high-frequency components of the measured signal. Using a Tektronix oscilloscope, only 2.5 points are needed for each measured signal cycle to reconstruct the waveform to the maximum extent. Some other oscilloscopes require more than 4 samples/cycle, that is, a 100MHZ bandwidth oscilloscope requires a sampling rate of at least 400MS/s for a single acquisition. Some oscilloscopes even require 10 points (linear interpolation technology) to ensure that the acquired signal is meaningful. [page]
34. What are the advantages and disadvantages of the so-called Gaussian response oscilloscope and flat response oscilloscope and what are their suitable fields?
Answer: There are no indicators of flat response and Gaussian response in the specifications of oscilloscopes. Similar comparisons or discussions may occur in oscilloscopes for the following reasons:
As we all know, oscilloscopes are time-domain instruments. Since Tektronix invented the first triggerable analog oscilloscope, the bandwidth of an oscilloscope has always been the most important indicator. It refers to the analog bandwidth of the preamplifier inside the oscilloscope. However, the definition of oscilloscope bandwidth is in the frequency domain, that is, the frequency point when the sine wave amplitude decays to the -3dB point. A complex high-speed signal contains a wealth of spectral components. If you need to accurately measure the signal, you must know the amplitude and phase of each of their spectral components, so the amplitude-frequency characteristic and phase-frequency characteristic of the oscilloscope are very important.
From the development in recent years, the bandwidth of digital oscilloscopes is getting higher and higher. From the launch of the TDS7000 4GHZ bandwidth oscilloscope by Tektronix in 2000, the launch of the TDS6000 6GHZ bandwidth oscilloscope in 2001, the launch of the TDS7704B 7GHZ bandwidth oscilloscope in 2003, to the recent TDS6804B 8GHZ bandwidth oscilloscope, the bandwidth has been increasing almost every year. When the bandwidth of the oscilloscope reaches several GHZ, it is increasingly difficult for the preamplifier, as an analog device, to ensure good amplitude-frequency and phase-frequency characteristics. Tektronix is the only company that has mastered this most critical technology. Some manufacturers cannot do this, so they have to use other methods to make up for the lack of bandwidth of analog devices and obtain higher bandwidth, and the frequency response curve will naturally change.
With the increasing number of various high-speed signals and faster signal rates, new requirements are put forward for real-time oscilloscopes. Some new technologies have also appeared in the digital oscilloscopes of oscilloscope manufacturers. The most significant is that the oscilloscope uses digital signal processing technology (DSP) to obtain better performance. DSP is mainly used in digital oscilloscopes, including:
enhanced bandwidth,
faster rise time
, gain and waveform calibration and improvement,
amplitude and phase improvement,
optical reference receiver normalization,
and Tektronix's third-generation oscilloscope (DPO) is the best embodiment of DSP technology. Reasonable use of DSP can improve the signal fidelity of oscilloscope testing.
However, the use of DSP technology will confuse every oscilloscope user, especially in terms of "whether bandwidth can be improved through DSP", "the bandwidth of the oscilloscope is analog bandwidth, what is the relationship with DSP technology", "is the current oscilloscope bandwidth analog bandwidth or DSP bandwidth?" "What are the negative effects of DSP technology?"
In the latest Tektronix TDS6804B 8GHZ bandwidth oscilloscope, the analog bandwidth is 7GHZ, and the bandwidth after DSP enhancement is 8GHZ. In order to ensure that every tester understands these two methods, the DSP bandwidth enhancement function can be turned on and off in TDS6804B. Tektronix tells every tester the advantages and problems brought by DSP enhanced bandwidth, helping testers understand the test results of analog bandwidth and DSP enhanced bandwidth, and better perform high-speed signal testing.
35. In addition to Gaussian response oscilloscopes and flat response oscilloscopes, are there oscilloscopes based on other responses?
Answer: The frequency response characteristics of the oscilloscope preamplifier are the most critical factor in determining the test results, which are determined by analog devices. The key lies in what method is used to obtain sufficient frequency response.
36. When I used oscilloscopes such as TDS744 and TDS745, I used passive probes (such as P6139A, bandwidth 500M). After I bought an active probe (P6237), the test results of the two were quite different from the test waveforms (especially when measuring high-frequency signals). From the probe parameters, we know that the input capacitance of the active probe is <1pF, while that of the passive probe is about 10pF. It seems that the test results of the active probe can better reflect the true situation of the signal. Since the passive probe has a great attenuation on high-frequency signals, what is the significance of the 500M bandwidth? How to choose to use an active or passive probe according to the test situation?
Answer: The typical bandwidth value of your P6139A probe plus Tektronix's 500MHz oscilloscope can still reach 500MHz, but as you said, its input capacitance is different. This capacitance will produce a load effect on the signal to be tested, causing signal ringing and shape changes. Therefore, using an active probe at this time can reflect the true situation of the signal. In fact, when using a probe, we should not only consider bandwidth, but all these factors should be considered when measuring high-frequency signals:
bandwidth/rise time
dynamic range
load effect
ground effect
resonance effect
Especially when using P6139A, you should also consider the influence of the ground wire. The ground wire on the probe will also cause ringing. When measuring high-frequency signals, the length of the ground wire should be shortened as much as possible.
In addition, the P6247 you use is an active differential probe, and the influence of common mode may also be a factor.
The main reason for choosing a passive probe is its large dynamic range. For example, P6139A can measure signals from millivolts to hundreds of volts, while P6247 can only measure +-8.5V signals. In addition, the price of active probes is also a factor.
37. During the experiment, the MOSfet exploded after the oscilloscope was grounded. Now the ground wire of the oscilloscope has been cut off. What is the reason?
Answer: In order to ensure personal safety during the test and obtain good measurement results, the ground wires of all probes of the oscilloscope are generally connected to the casing and connected to the ground wire of the oscilloscope power cord. Therefore, when you measure the waveform of the MOSFET tube in the power supply, if any point is not ground, there will be problems, as shown in the figure below.
Cutting the ground wire can prevent short circuit problems in the MOSFET tube test, but it will also bring some other test problems, such as the oscilloscope case being charged, and the oscilloscope case distribution parameters affecting the measured signal. The solution is to use a differential probe, such as Tektronix's P5205, which can measure the differential signal when neither of the two test points is ground.
38. When using an oscilloscope to capture data, it is found that the stored text only contains the data on the current screen, and the time interval is based on resolution. How to use software to process data in real time (matlab?), how to capture more data?
Answer: Tektronix oscilloscopes use a compressed screen display style, that is, the waveform displayed on the screen is all the collected data. With the multiViewZoom function of TDS5000B, all waveforms can be easily displayed.
Tektronix TDS5000B, TDS6000, TDS7000B, TDS8000B series oscilloscopes all use a completely open WINDOWS platform and support all current popular tools, such as Matlab, LabView, VB, VC, .NET, MicroSoft Office VBA, etc., which can flexibly perform data analysis and processing.
These analysis tools can also be directly installed in the oscilloscope to form an instrument that integrates data acquisition, analysis, display, and processing. To collect more data at a time, the oscilloscope needs to be equipped with a deeper storage depth, such as the TDS5000B series general-purpose oscilloscope, which can support up to 16M memory.
39. What are the factors that affect the working speed of the oscilloscope?
Answer: In fact, the principles of any oscilloscope are similar. The front end is a data acquisition system and the back end is computer processing. There are two main aspects that affect the speed. One is the data transmission from the front-end data acquisition to the back-end processing, which generally uses the PCI bus. This is a transmission bottleneck, but there are new technologies that can break through it; the other is the back-end processing method. Improving the processing speed can be achieved through data packet sharing.
40. Our applications usually capture 2M or even more data for analysis, and the sampling rate is usually as high as 10GS/S, but it always seems very slow when performing parameter testing and FFT analysis. Why?
Answer: The processing speed is naturally slow when the amount of data is large. In order to obtain high-speed real-time FFT analysis of large amounts of data, a dedicated FFT processor must be used, but the cost is high.
41. When using Tektronix's TDS2014 digital oscilloscope to capture the timing of a parallel port, a very strong 50Hz AC can always be measured, but no signal can be measured. However, the ground of the oscilloscope is consistent with the ground of the measured parallel port. What should I do?
Answer: You can start from the following aspects:
① Check whether the oscilloscope is well grounded or isolated by an isolation transformer;
② Whether there is a strong 50Hz signal induction nearby;
③ In a strong interference environment, you should pay attention to whether the parallel port's driving capability and operating frequency are suitable for the test operation selection. If you only see a 50Hz interfering sine wave, and the waveform is relatively regular, you should consider that the parallel port may not be working;
④ Check if the probe tip is damaged;
⑤ It is recommended to unplug all unnecessary peripherals, which may also come from the monitor;
⑥ If the oscilloscope has been used for a long time, you should consider whether the bottom line is normal, that is, the small clip. Remove the probe and measure it with a multimeter.
42. To solve the problem of power supply interference, you want to measure the situation where the interference signal of the total power supply is connected to the power supply of the weak signal amplifier. As a result, even if the oscilloscope probe is connected to the ground, there is an interference signal, no matter where it is measured. The interference signal is audio. Why is this?
Answer: The following issues need attention:
① The grounding problem of the oscilloscope. The reference ground wires of the oscilloscope housing and the probe are connected to the ground wire. Therefore, good grounding is the primary condition for measuring interference.
② The interference introduced by the reference ground wire of the oscilloscope. Since ordinary probes usually have a ground wire, they will form an interference path similar to a loop antenna with the point to be measured, introducing relatively large interference. Therefore, to minimize this interference, you can remove the probe cap and do not use the ground wire led out from the probe. Instead, directly use the probe tip and the ground inside the probe to contact the point to be measured for measurement.
③ Use differential measurement methods to eliminate common-mode noise. Tektronix provides a series of differential probes, such as the ADA400A specifically for small signals, which can measure hundreds of microvolts, and the P7350 for high-speed signal measurement provides a bandwidth of up to 5GHz.
④ Many Tektronix oscilloscopes provide a high-resolution acquisition (Hi-Res) signal capture mode, which can filter out random noise superimposed on the signal.
43. Sometimes in EMC test, the indicator disappears for a short time. When using an oscilloscope for testing, it is found that the entire screen of the oscilloscope shakes during the test. The test item is EFT (transient pulse train immunity test). How to explain and eliminate this phenomenon in the test?
Answer: EFT sometimes interferes with the oscilloscope and causes false triggering. You can try to use the high-frequency suppression trigger mode of the oscilloscope, limit the bandwidth of the oscilloscope, etc.
44. Why can't the oscilloscope sometimes capture the amplified current signal?
A: If the signal does exist, but the oscilloscope can sometimes catch it and sometimes not, this may be related to the settings of the oscilloscope. Usually, if you can set the oscilloscope trigger mode to Normal, set the trigger condition to edge trigger, and adjust the trigger level to an appropriate value, and then set the scan mode to single mode, if this method still does not work, there may be something wrong with the instrument.
45. How can the new digital oscilloscope be used for single-chip microcomputer development?
A: In the process of single-chip microcomputer circuit development, generally speaking, there are no problems with the components and chips themselves. The problem is often that the communication between them is different from what is expected. In single-chip microcomputers, common buses are SPI, I2C, USB, LIN, CAN, 54621A and 54621D oscilloscopes themselves support the trigger function of serial signals, and can directly debug the communication on the serial bus. In addition, if you use DSP combined with MCU to develop a circuit board, it may involve software and hardware joint debugging. At this time, you can use the digital logic channel of 54621D to connect to the control line or data and address line to determine whether the circuit can work normally under specific operating conditions or subroutine operation. Moreover, its 2M-point storage depth per channel is very helpful in analyzing the cause of the problem, observing the serial signal for a long time, observing the handshake timing, etc. And its amplification function can amplify the signal by tens of thousands of times to observe the details.
The price of 54621A should be around US$2500, and the price of 54621D should be less than US$4000. You can visit http://www.agilent.com/find/mso http://www.agilent.com/find/test to download many related application articles.
46. Do the new digital oscilloscopes 54621A and 54621D have any effect on the different signals and different rates of the (Inter-IC) bus during testing?
Answer: The I2C Bus signal generally works at a rate of no more than 400Kbit/s. Recently, chips with several Mbit/s have also appeared. When setting the trigger conditions for 54621A and 54621D, you do not need to consider the impact of different rates. However, for other buses, such as the CAN bus, you must first set the actual current working rate of the CAN bus on the oscilloscope so that the oscilloscope can correctly decode the protocol and trigger correctly.
47. In addition to the oscilloscopes 54621A and 54621D, what other instruments can detect and analyze Inter-IC bus signals?
Answer: If you want to conduct further analysis on the Inter-IC bus signal, such as protocol-level analysis, you can use Agilent's logic analyzer, but relatively speaking, the price is higher than that of 54621A/D.
48. What kind of signals are suitable for testing various trigger applications of digital oscilloscopes, such as edge trigger, glitch trigger and pulse width trigger?
Answer: ① Edge trigger, edge trigger, you can set the trigger level, rising edge or falling edge. Edge trigger is also called basic trigger.
② Advanced trigger, which includes various trigger functions. You can set the corresponding trigger conditions according to the characteristics of the measured signal and locate the waveform of interest.
Advanced trigger is the key to circuit debugging. During the circuit debugging process, if you do not know the possible problems of the measured signal in advance, you can first use the Tektronix digital phosphor oscilloscope to use the 400,000/second waveform capture speed to quickly find various problems in the circuit, and then use different advanced trigger functions to locate the details of the fault, which can shorten your debugging cycle.
49. Regarding glitch measurement, I have consulted relevant technicians before, and the answer I got was that the minimum glitch that an oscilloscope can capture is the sampling rate of the oscilloscope. Do all oscilloscopes follow this rule? Will the pre-filter of the oscilloscope have no effect on it at this time?
Answer: It cannot be asserted that all oscilloscopes are like this. For example, some oscilloscopes reach 1GS/s, and the bandwidth is only 60MHz. Obviously, it is impossible to capture a 1ns glitch. In fact, the ability to capture glitches depends not only on bandwidth and sampling rate, but also on waveform capture rate, that is, the number of waveforms that can be captured per second. For details, please refer to Tektronix's application article on DPO.
50. How to eliminate glitches when using an oscilloscope?
Answer: If the glitches are inherent in the signal itself, and you want to use edge triggering to synchronize the signal (such as a sine signal), you can use the high-frequency suppression trigger method, which can usually synchronize the signal. If the signal itself has glitches, but you want the oscilloscope to eliminate the glitches and not display the glitches, it is usually difficult to do so. You can try to use the bandwidth limitation method, but you may accidentally filter out part of the information of the signal itself. If you use a logic analyzer, generally speaking, using the state acquisition method, some glitches collected in the timing mode will not be visible.
51. In actual work, when encountering sudden glitch signals, how to capture and test them?
A: For example, when we are testing the clock, we often encounter occasional glitch signals, which will cause malfunctions in our circuits. Therefore, capturing the signal becomes the key to the test. Since we cannot determine whether the glitch is positive or negative in advance, we must first use the digital phosphor function of the TDS5000 oscilloscope, that is, the fast waveform capture mode combined with infinite afterglow to view the glitch characteristics, and then use the advanced trigger function of the oscilloscope - pulse width triggering according to the signal characteristics, such as: triggering when the pulse width is less than the normal clock pulse width.
52. What are the applications of glitch/pulse width triggering?
A: There are generally two typical applications for glitch/pulse width triggering. One is to synchronize circuit behavior, such as using it to synchronize serial signals, or for applications with very serious interference, it is impossible to use edge triggering to correctly synchronize signals. Pulse width triggering is an option; the other is to find abnormal phenomena in the signal, such as narrow glitches caused by interference or competition. Since the abnormality appears occasionally, it must be captured by glitch triggering (another method is peak detection, but the peak detection method may be limited by its maximum sampling rate. At the same time, it is generally visible but not measured). If the pulse width of the object under test is 50ns, and there is no problem with the signal, that is, there is no signal distortion or narrower due to interference, competition, etc., the edge trigger can be used to synchronize the signal without using glitch trigger. Many users set the pulse width trigger to 10ns ~ 30ns. Fortunately, 5462x and 5464x are rare instruments in the industry that can complete this operation. If you want to verify whether there are abnormal pulses in the 10MHz square wave, including pulses much narrower than 50ns, you will use pulse width or glitch trigger, and you may use the 5ns setting.
53. Does Agilent's digital oscilloscope have DPO function?
Answer: DPO is a special term. Only one oscilloscope company uses this term. Agilent's corresponding function is called MegaVision. The similarities with DPO are: ① It can directly detect abnormal phenomena in the signal. ② The waveform capture rate is much higher than that of ordinary digital storage oscilloscopes. Differences: ① After discovering an abnormal signal, MegaVision can directly amplify the abnormality and observe the signal details. ②The real-time sampling rate of MegaVision oscilloscopes exceeds the limit of 1.25GSa/s and can reach 2GSa/s (such as 5464xA/D oscilloscopes) or even higher. ③MegaVision oscilloscopes are optimized for applications that require deep storage. When the oscilloscope storage depth is >10K, or even 100K, 2M, its waveform refresh rate is the industry's leading.
54. If the bandwidth is determined based on the signal rise time, is the principle of determining the sampling rate based on the bandwidth just to achieve no sampling aliasing error?
Answer: Determine the sampling rate after determining the bandwidth. Some formulas in the industry do determine the principle of the sampling rate in order to achieve no sampling aliasing error, but it is a general evaluation statement. It depends on the characteristics of your test object, because the highest indicators are often given under specific conditions and may not meet your test application.
55. How does an oscilloscope display the waveform between two sampling points?
Answer: There are many display modes for oscilloscopes: point display, sine interpolation display, and straight line connection display; the default display mode of an oscilloscope is usually vector connection display, and some oscilloscopes only support straight line connection; whether it is straight line connection or sine interpolation, the information provided between two actual sampling points is not actually collected, because the straight line connection may cause a sudden change in the display, such as collecting a point at the peak of a sine wave and collecting a point at the trough on both sides, a triangle wave will be displayed, while the sine interpolation display is still a sine wave. Therefore, some application articles say that: using straight line connection, the sampling rate requirement is higher, such as 10 times (to truly reproduce the waveform); using sine interpolation, the sampling rate requirement is slightly lower, and some articles say that 2.5 times is enough. In engineering, it is generally said to be more than 4 times, and there are also 5 times and 6 times.
56. High-speed signal characteristics on PCB board: 156.25MHZ differential clock signal, Rise/Fall Time (20%～80%) <100ps, jitter tolerance (pp<30ps, RMS<2ps), skew (+ vs.-) <20ps, what bandwidth oscilloscope is needed for accurate measurement? What is the measurement error?
Answer: For 156.25MHz differential clock signal, Rise/Fall Time (20%～80%) <100ps, if you want to accurately test the rise time, such as 3% test accuracy, 0.4/100ps *1.4 = 5.6GHz bandwidth oscilloscope and its probe system, if 10% accuracy is acceptable, 0.4/100ps*1.2 = 4.8GHz bandwidth oscilloscope and its probe system. Note that if you use a differential probe, you must ensure that the bandwidth of the entire oscilloscope is 5.6GHz from the point being measured. Fortunately, Agilent has launched a differential probe with a bandwidth of 7GHz. At the same time, the actual rise time indicator of the 54855A itself is 65ps, and the manual gives an indicator of 72ps. jitter tolerance (pp<30ps,RMS<2ps), to accurately measure jitter indicators, the jitter indicators of the oscilloscope itself must be higher. The trigger jitter indicator of 54855A itself is 1ps RMS, which is 7 times better than similar products in the industry. Another related indicator is Delta Time meas. Accuracy (peak) is ± [ (7.0 ps) + (1 x ppm * |reading|) ], which is more than 2 times better than similar products. This is related to the fact that it actually uses a 20GSa/s A/D, which eliminates the error caused by using multiple (10GSa/s A/D or 5GSa/s A/D) to piece together a 20GSa/s.
57. When choosing an oscilloscope, the bandwidth is generally considered. So, under what circumstances should the sampling rate be considered?
Answer: It depends on the object being measured. Under the premise of meeting the bandwidth, it is hoped that the minimum sampling interval (the inverse of the sampling rate) can capture the signal details you need. There are some empirical formulas for sampling rate in the industry, but they are basically derived for the bandwidth of the oscilloscope. In practical applications, it is best not to use an oscilloscope to measure signals of the same frequency. If you are selecting a model, for a sine wave, choose an oscilloscope with a bandwidth that is 3 times the frequency of the sine signal being measured. If the bandwidth is above 3 times, the sampling rate is 4 to 5 times the bandwidth, which is actually 12 to 15 times the signal. For other waveforms, ensure that the sampling rate is sufficient to capture signal details. If you are using an oscilloscope, you can verify whether the sampling rate is sufficient by the following methods:
stop the waveform and enlarge the waveform. If you find that the waveform has changed (such as certain amplitudes), the sampling rate is not enough, otherwise it will not be a problem. You can also use point display to analyze whether the sampling rate is sufficient.
58. A 100MHz analog oscilloscope can see the parasitic waveform more clearly, but a 100MHz digital oscilloscope cannot (it can only see the waveform in bold)?
Answer: This phenomenon is related to the oscilloscope display. The traces seen on an analog oscilloscope are generally thinner. It directly hits the voltage on the screen through a vertical deflector, and the scanning rate and waveform refresh rate are very fast. Digital oscilloscopes quantify waveform voltages through A/D, store them in memory, and display them after processing. The display resolution of digital oscilloscope screens is limited, usually 640 points or 1000 points. If you set the storage depth (record length) of the oscilloscope to 10K or 2M, this means that the information of 10K or 2M points in memory must be reflected through 640 points or 1000 points. No matter how good the algorithm is, it will bring certain display errors. The degree of waveform bolding is related to the storage depth. These problems are unique to digital oscilloscopes. In addition, the default display mode of digital oscilloscopes is vector display mode, which means that some points will be inserted between two sampling points using linear algorithms or sine interpolation algorithms. Analog oscilloscopes do not have these problems. You can try to change the oscilloscope record length to 500 points, and change the vector display to point display, observe the actual data obtained by the digital oscilloscope each sampling, adjust the time base, and you can clearly see these points. Even if you use vector display, the line will become thinner. From the perspective of the instrument, when measuring small signals, the results of using a 1:1 probe may be better than those of a 10:1 probe. In addition, analog oscilloscopes do not have the concept of sampling rate, but only the concept of scanning rate. When using a digital oscilloscope, the sampling rate often needs to be considered.
59. Which one is more advantageous when observing the details of the waveform (for example, observing parasitic waveforms below 1% when crossing the zero point and the peak)?
Answer: When observing parasitic waveforms below 1%, whether it is an analog oscilloscope or a digital oscilloscope, the observation accuracy is not very good. The vertical accuracy of an analog oscilloscope may not be higher than that of a digital oscilloscope. For example, the vertical accuracy of an analog oscilloscope with a bandwidth of 500MHz is +/-3%, which is not more advantageous than that of a digital oscilloscope (usually 1~2% accuracy). In addition, the automatic measurement function of a digital oscilloscope is more accurate than the manual measurement of an analog oscilloscope for details.
60. Digital oscilloscopes generally provide online display of the RMS value. What is its accuracy?
A: Many people use the number of A/D bits to measure the amplitude measurement accuracy of the oscilloscope. In fact, it will change with the bandwidth of the oscilloscope you use, the actual sampling rate setting, etc. If the bandwidth is not enough, the amplitude measurement error itself will be very large. If the bandwidth is sufficient and the sampling setting is very high, the actual amplitude measurement accuracy will not be as good as the accuracy when the sampling rate is low (you can sometimes refer to the user manual of the oscilloscope, which may give the actual effective number of A/D bits of the oscilloscope at different sampling rates); in general, the accuracy of oscilloscopes in measuring amplitudes, including RMS values, is often not as good as that of multimeters. Similarly, when measuring frequencies, it is not as good as frequency counters.
61. How to capture and reproduce instantaneous signals that disappear in a flash?
Answer: Set the oscilloscope to single acquisition mode (set the trigger mode to Normal, set the trigger condition to edge trigger, adjust the trigger level to an appropriate value, and then set the scan mode to single mode). If you are using Agilent 5462xA/D, 5464xA/D, 5483xB/D, 5485xA, these instruments all support the MegaZoom function, that is, you can observe the overall signal while zooming in on the local details, either by moving the screen or by using the dual time base display function. Note that the oscilloscope's memory depth will determine the time that can be collected and the maximum sampling rate that can be used.
62. Which Agilent oscilloscope can test a carrier signal with a frequency of 500M?
Answer: If you only measure the carrier signal itself, usually the carrier signal is a sine wave, it is recommended to use a 1.5GHz oscilloscope (Agilent 54845B), use a BNC cable to connect the object to be measured, and you can get a rise time measurement accuracy of ~94.6%. If a probe must be used, the 1157A active probe (2.5 GHz bandwidth) is recommended. If a 500 MHz bandwidth oscilloscope is used, even with a BNC cable, the best-case amplitude measurement error is 29.3%, and the rise time measurement accuracy is 58.6%.
63. If the oscilloscope is rated at 60MHZ, can it be understood that it can measure up to 60MHZ?
Answer: A 60MHz bandwidth oscilloscope does not mean that it can measure 60MHz signals well. According to the definition of oscilloscope bandwidth, if a 60MHz sine wave with a peak-to-peak value of 1V is input to an oscilloscope with a 60MHz bandwidth, a 0.707V signal will be seen on the oscilloscope (30% amplitude measurement error).
64. Why can't a 4.1943MHZ square wave be measured with an oscilloscope rated at 60MHZ?
Answer: If the square wave is to be tested, the reference standard for selecting the oscilloscope is the rise time of the signal. If the oscilloscope bandwidth = 0.35/signal rise time * 3, the rise time measurement error is about 5.4%.
The probe bandwidth of the oscilloscope is also very important. If the bandwidth of the system composed of the oscilloscope probe and its front-end accessories is very low, the bandwidth of the oscilloscope will be greatly reduced. If you use a 20MHz bandwidth probe, the maximum bandwidth that can be achieved is 20MHz. If you use a connecting wire at the front end of the probe, the performance of the probe will be further reduced (but it should not have much impact on the ~4MHz square wave because the speed is not very fast).
In addition, check the oscilloscope manual. Some manufacturers' newly launched oscilloscopes will have their actual bandwidth sharply reduced to <=6MHz under the 1:1 setting. For a ~4MHz square wave, its third harmonic is 12MHz and its fifth harmonic is 20MHz. If the bandwidth is reduced to 6MHz, the signal amplitude will be greatly attenuated. Even if you can see the signal, it is definitely not a square wave, but a sine wave with attenuated amplitude. Of course, there may be many reasons for not being able to measure the signal, such as poor probe contact, but this phenomenon can be easily eliminated. It is recommended to connect a function generator with a BNC cable to check whether there is any problem with the oscilloscope itself and the probe. If there is any problem, you can contact the manufacturer directly.
65. How to measure the stability of the clock?
A: If you are using 5483xB/D, 548xxA, 5484xB or 5485xA, you can use the standard configuration histogram method to measure the jitter of the clock edge or amplitude. For details, please refer to Agilent's application article: "Jitter Analysis Techniques Using an Agilent Infiniium Oscilloscope" (P/N: 5988-6109EN), which can measure the jitter in the worst case. For 5485xA, if you want more powerful jitter analysis functions, it is equipped with special jitter analysis software, which provides very powerful jitter analysis. For details, please refer to the Datasheet of the 5485x oscilloscope. For more detailed information, please call Agilent.
66. What are the methods and techniques for accurately measuring period jitter in PLL using Agilent oscilloscopes?
A: If you are using 5483xB/D, 548xxA, 5484xB and 5485xA, you can use the standard configuration of the histogram method to measure the jitter of the clock edge or amplitude. For details, please refer to Agilent's application article "Jitter Analysis Techniques Using an Agilent Infiniium Oscilloscope" (P/N: 5988-6109EN), which can measure the jitter in the worst case. For 5485xA, if you want a more powerful jitter analysis function, it is equipped with a special jitter analysis software, which provides very powerful jitter analysis. For details, please refer to http://www.agilent.com/find/test 5485x oscilloscope datasheet. For more detailed information, please call Agilent. Reminder: When using an oscilloscope, pay attention to whether its jitter-related indicators meet the test requirements, such as the trigger jitter indicators of the oscilloscope itself, etc. At the same time, pay attention to the use of different probes and probe connection accessories. If the system bandwidth of the oscilloscope cannot be guaranteed, the measurement results will be inaccurate.
67. How to use an Agilent oscilloscope to measure the Settle time of a PLL?
Answer: You can use an Agilent 548xx series oscilloscope + USB-GPIB 82357A adapter + software options to complete it. You can also use Agilent's lower-priced modulation domain analyzer to complete it.
68. Design a PLL, how to measure the dead zone of the PFD (phase detector)?
Answer: You can connect one channel of the oscilloscope to the reference signal and the other channel to the feedback signal, and set the trigger condition of the oscilloscope to the setup and hold time trigger. At this time, while adjusting the setup and hold time setting of the oscilloscope, adjust the reference signal until it loses lock. At this time, the setup and hold time setting corresponds to your PFD dead zone. Theoretically, it is believed that the loss of lock will occur at two moments: one is at the initial working time, when the phase difference (frequency difference) of the two signals exceeds the capture bandwidth of the PLL; the other is during the tracking process, when the feedback signal changes too much, causing the phase difference of the two signals to exceed the tracking bandwidth of the PLL and the lock will be lost. All Agilent 548xx series oscilloscopes can complete this measurement (under the premise that the bandwidth is sufficient).
69. How to test optical signals using Agilent equipment?
Answer: Agilent has a full set of test solutions to measure optical signals, from light sources, spectrometers, optical multimeters, optical oscilloscopes, optical wavelength meters, etc. If you want to use a real-time oscilloscope to measure optical signals, you can use an optoelectronic converter combined with an oscilloscope to complete the measurement.
70. How to use an oscilloscope to measure power supply ripple?
Answer: You can first use an oscilloscope to capture the entire waveform, then zoom in on the ripple part of interest to observe and measure (automatic measurement or cursor measurement), and also use the oscilloscope's FFT function to analyze from the frequency domain. [page]
Usually, if the details of the object being measured (amplitude, frequency, etc.) are not clear, you can use the "AutoScale" button to observe the general signal, and then adjust the horizontal control knob and the vertical control knob to get the best display (for example, the amplitude should be displayed as full screen as possible), and then use the Zoom function to fully enlarge the waveform. When measuring power supply ripple, you can use the Zoom function to enlarge the ripple part for analysis; in addition, you may consider analyzing the power supply from the frequency domain perspective to observe its harmonics and noise. To this end, you can let the oscilloscope display as many cycles of signals as possible, use the oscilloscope's storage depth as much as possible, and set the sampling rate to an appropriate value to ensure that the waveform is not distorted. The frequency resolution obtained in this way is the sampling rate divided by the current storage depth setting, and observe the amplitude difference between each harmonic and the fundamental wave. In addition, if you use MatLab software, you can use the powerful functions of MatLab software to conduct a more in-depth analysis of the captured signal data. 546xx and 548xx are both equipped with software that connects to a computer as standard, which allows data to be directly transferred to the computer for further analysis. Of course, Matlab software can also be directly installed in the 548xx.
If the circuit parameters are already known, the oscilloscope settings can be directly adjusted to allow it to work at a suitable sampling rate and vertical scale.
71. The ripple of the output voltage of a switching power supply is an important indicator. How to correctly use an oscilloscope to measure this indicator?
Answer: The definition of ripple is a clutter signal containing periodic and random components attached to the DC level, which is called PARD (Periodic And Random Deviation) in English. It is defined as the peak-to-peak value of the clutter. Things to note when measuring ripple: The ground wire of the oscilloscope probe will cause a lot of ripple. The ground wire should be unplugged and the probe internal wire should be used for measurement. Of course, the best measurement method is to use a 50 ohm terminal resistor and connect it directly to the oscilloscope with a BNC cable. It should be noted that the 50 ohm resistor should take power consumption into consideration, and a high-power resistor may be required. Relevant standard requirements, such as whether to separate periodic power frequency ripple and switching ripple, high-frequency noise, etc. Another example is whether the measurement frequency should be limited to below 20MHz.
72. When measuring ripple, a large part of it is a periodic spike pulse of 50 Hz. The larger the load current, the larger the pulse amplitude. What are the specific solutions?
Answer: In the Tektronix power measurement system, when measuring ripple, we can choose the power frequency ripple test or the switching ripple test, so that the ripple of irrelevant frequency can be automatically filtered out. For example, if you choose to test the 200KHz ripple, the oscilloscope will automatically filter other frequency components.
73. When measuring ripple, how to remove the noise on the ripple, such as power frequency noise?
Answer: The noise on the ripple can be removed by the high-resolution capture mode of the TDS5000 oscilloscope in the capture mode. These random noises. There are two types of ripples: one is the power frequency, 100HZ, and the other is the switching ripple. The TDSPWR2 launched by TEK can separate these two ripples and measure them separately to get the results.
74. When accurately testing the ripple and noise of the switching power supply, do you have to go to a special laboratory?
Answer: Of course, it would be ideal if there is a special laboratory for ripple measurement. When this condition is not met, the following issues should be noted:
① The oscilloscope should have a good grounding;
② If the measurement standard has bandwidth limit requirements, the 20MHz bandwidth limit in TDS430A should be turned on;
③ Use the AC coupling of the oscilloscope;
④ Use BNC cables and use the 50 ohm input impedance range of TDS430A for measurement (a 50 ohm high-power load, BNC adapter or test fixture may be required at this time)
To improve the measurement accuracy, the oscilloscope probe should not be used, and the ground wire of the oscilloscope probe will introduce relatively large noise.
75. How to use an oscilloscope to measure the output ripple value of some low-ripple power supplies? For example, when measuring the output ripple of 1.8V, the nominal output ripple is generally less than 20mV. How to use an oscilloscope to verify it? Even if the ordinary oscilloscope probe is directly connected to the probe ground clip, the noise is 20 to 30 millivolts.
Answer: This problem is very representative. A voltage differential probe with a high common-mode rejection ratio should be used, which can work in a high-noise environment.
76. How to use a digital oscilloscope to view and read the cycle of the displayed waveform?
Answer: All digital oscilloscopes support waveform cycle measurement. From the perspective of improving test accuracy, if you are using 5462x/5464x (except 54645), you can select Counter in its measurement parameters, and its embedded hardware frequency counter will be activated for accurate frequency measurement (5 digit). If you are using other types of oscilloscopes, try to let the oscilloscope screen display a cycle of the signal, and the amplitude should be as full scale as possible. At this time, the measurement accuracy is generally good. You can use the oscilloscope's automatic measurement function or use the cursor to measure manually.
77. I encountered a problem during development. I added functions to the prototype machine to detect the prototype's audio frequency, data output, trigger signal, etc. The test results are almost the same as the design results. Why is the prototype sound clear and accurate, while the sound of the finished product is sometimes acceptable, but sometimes not?
Answer: The sound of the actual object being tested is sometimes acceptable and sometimes not, but there is no problem with the waveform display on the oscilloscope, or the data displayed by the oscilloscope is far different from the data on the object being tested. This is often caused by the oscilloscope and your object being measured not being synchronized. You can try the following methods:
Sound signals are usually low-speed signals. You can let the oscilloscope work in scrolling mode. When observing the signal, manually stop waveform acquisition and analyze it.
Observing sound signals in the time domain is often not comprehensive. Agilent's dynamic signal analyzer is a better choice in many cases, but if you don't have this instrument, you can combine the oscilloscope's FFT function to observe from the frequency domain.
Try using the oscilloscope's trigger function. If you have a mixed signal oscilloscope (54xxxD), you can define trigger conditions in combination with its logic channels (such as state triggers and sequential triggers similar to logic analyzers).
78. How to perform clock jitter testing with a tds3012 oscilloscope?
Answer: Tektronix's open platform oscilloscopes (such as TDS7000, TDS5000) have special jitter measurement software that can perform comprehensive jitter measurements (such as Rj, Dj, etc.). In TDS3012, you can only perform relatively long-term cumulative measurements on signals through infinite persistence. In addition, jitter measurement is only required for clocks with relatively high frequencies. The general principle of measuring signals with an oscilloscope is that the bandwidth of the oscilloscope should be 5 times the highest frequency of the signal. If the square wave with a relatively fast rise time may require an oscilloscope bandwidth that is 10 times or even higher than the signal frequency. Therefore, it is recommended to use an oscilloscope with a higher bandwidth and development platform.
79. How to use an oscilloscope to measure the power factor in an AC/DC switching power supply?
Answer: In fact, using an oscilloscope to measure the power factor is to measure the phase difference between voltage and current, that is, cosφ. At the same time, the Tektronix TDS5000 power test system also automatically measures the relevant parameters of PFC (such as THD, True Power, Apparent Power, Power Factor, etc.).
80. The FFT function of the Tektronix oscilloscope can see the frequency and amplitude of the radiation of the switching power supply, but is the amplitude value here the same as the value of the certification center? If not, how to convert it? Moreover, if you select different V/DIV when viewing the waveform, there will be different amplitudes in the FFT state. Is this normal? ---The model I use is TDS1012.
Answer: The amplitude measured by the FFT function of the oscilloscope can only be used as a qualitative analysis, not a quantitative analysis, so it is only of reference value. If you want to analyze the spectrum amplitude, you can choose the Blackman-Harris window, which will have a better effect; when converting V/div, it will definitely affect the amplitude of the FFT, because this is limited by the resolution of the ADC of the oscilloscope itself. Therefore, in order to improve the measurement accuracy, it is generally chosen to fill the entire screen with the waveform as much as possible (but never exceed the screen), that is, to choose a smaller V/div gear.
81. What type of oscilloscope can effectively improve design efficiency?
Answer: At the current stage of oscilloscope development, data analysis has been raised to an important position. Using an oscilloscope is not only to observe waveforms during debugging, but more importantly, it can analyze and calculate device parameters in design to help everyone optimize the design plan. What kind of oscilloscope is most suitable should be determined in combination with the signal you want to observe and analyze.
82. How to use an oscilloscope to test video parameters (including video output level, horizontal resolution, brightness amplitude-frequency response, chroma amplitude-frequency response, brightness signal-to-noise ratio, chroma signal-to-noise ratio, brightness nonlinear distortion and other video parameters)?
Answer: Tektronix TDS3000B series oscilloscopes plus TDS3VID or TDS3SDI and TDS5000 series oscilloscopes all provide powerful video measurement functions, even including analog HDTV functions, and built-in vector oscilloscope capabilities to help you analyze various video parameters.
83. At the high frequency end, how to determine the impact of the impedance of the oscilloscope probe itself on the signal?
Answer: The probes of the oscilloscope have specific indicators. You can refer to the equivalent impedance-frequency diagram of the probe to determine the equivalent impedance of the probe at the frequency point. Regarding probes, Tektronix has a special article called "Probe ABC".
84. Why is the waveform ringing of a 30MHz clock tested by a Tektronix oscilloscope much larger than that of an Angelen (the oscilloscope probe is 250MHz)?
A: When measuring state transition, you only need to use the automatic trigger mode of the oscilloscope to set the voltage and current waveforms to a more ideal display mode. If you use TDS5000, you can also adjust the resolution knob to set the sampling rate to a suitable level (generally about 10 times the signal frequency). Then use PWR2 software to automatically calculate the measured data. For MOSFET, we choose Vds and Ids as the measured signals, and for IGBT, we choose Vce and Ice as the measured signals.
When using a digital oscilloscope to test a switching power supply, can you pre-set limit parameters (such as test time, number of samples per time)? How to use a Tektronix oscilloscope to test the state transition of a switching power supply. Connection method (with examples), oscilloscope button settings, and necessary precautions.
85. When designing a soft-switching PWM converter (such as a PWM half-bridge switching converter), how do you use an oscilloscope to observe the MOSFET Vt/It trajectory?
A: First, the oscilloscope must have a time delay correction function between channels, so that basic accuracy can be guaranteed when performing related mathematical operations. Use a high-voltage differential voltage probe and a current probe for measurement. The power test solution launched by TEK can dynamically observe the entire working process of MOSFET.
86. The selection of output capacitor and output inductor should be determined according to the power supply demand of the load. Should the L and C values be applied according to the formula determined on the datasheet? If the value calculated according to the formula has problems in actual application, what should we replace it based on?
Answer: The calculation formulas of output chokes and output filter capacitors of different topologies are different. You should choose the appropriate calculation formula according to the circuit structure you choose. The size of the output capacitor is mainly determined by how many millivolts the output ripple voltage should be suppressed to. This requires calculating the ESR, and then you can choose it according to the DATASHEET provided by the manufacturer. However, when selecting a capacitor, you must also consider the change of load, current change range, output inductance, etc., because they will change the characteristics of the capacitor.
87. At present, HID hernia lamps have been widely used in some high-end car headlights, but in the high-voltage circuit design of HID lamp ballasts, it is found that the high-voltage recovery speed is not fast enough, which sometimes causes poor lighting. How to solve it?
Answer: HID lamps generally have a secondary breakdown process, and then the headlights tend to a stable working state; first of all, the secondary breakdown must be effectively controlled to ensure its stable operation. To measure the secondary breakdown, you only need to use the long record length of TDS5000, perform a single trigger to capture its waveform, and then measure the peak voltage and pulse width of the primary and secondary breakdown respectively, and then measure the time between the two breakdown pulses. According to the actual situation, see if the above parameters meet the design requirements.
88. If you use a probe and a virtual instrument, you can display the waveform on the PC. At the same time, various calculations can be easily achieved. What is the essential difference between the TEK5000 series and virtual instruments?
Answer: Although DS5000 is an oscilloscope based on Windows 2000, it is actually divided into two important parts. First, it has a real oscilloscope acquisition and processing part. The data processing of this part is carried out by a professional processor of the oscilloscope itself, while the computer platform of Windows 2000 only performs some background analysis and calculation processing on the data collected by the oscilloscope (internal communication through the PCI bus), which has nothing to do with the display of the oscilloscope itself.
The so-called virtual instrument (mostly PC plug-in card type) collects external signals into the computer through a data acquisition card (generally very slow) and processes the data through the computer's own CPU. It is a cheap solution, but its fatal weakness is that it has no traceability (it is too affected by the computer host, and the test results caused by different hosts have large errors). We know that the consistency of the test instrument is the key to determine the success or failure of the test results.
89. How to reduce the heat loss of DC-DC transformers? What issues should be paid attention to when designing transformers? What are the requirements for the peripheral circuits of transformers?
Answer: The principle of flux reset should be followed. To design a transformer, you need to select the core specifications and size, calculate the duty cycle, magnetic induction increment, and the number of turns of the primary and secondary sides. In the experiment, check the worst-case magnetic saturation.
90. The most difficult problem often encountered in the design of switching power supplies is the efficiency problem. The efficiency of the whole machine depends largely on the loss of the switch tube. After our circuit and device are selected, it is very important to measure the switching waveform of the switch tube. We can judge and improve the working state of the switch based on its data. So how should we operate correctly and pay attention to what problems when using an oscilloscope to perform this test?
Answer: There are two major themes in switching power supplies: improving efficiency and improving reliability. Efficiency requires measuring losses, which are mainly concentrated on the switch tube and magnetic components. For this reason, we should use an oscilloscope to measure the turn-on loss, cut-off loss, and conduction loss. Similarly, the core loss and dynamic inductance of transformers and inductors can be measured.
91. In actual work, it is necessary to test and analyze the switch oscillation signal, video signal, etc. How to do it?
Answer: TEK's TDS5000 series oscilloscope can easily measure and analyze these two types of signals.
For the driving signal of the switching power supply you mentioned, our TDSPWR2 provides four types of analysis: duty cycle trend analysis, switching frequency trend analysis.
Width and cycle trend analysis: TDS5000 oscilloscope has more abundant video triggers, can apply multiple formats, can trigger the field separately and in parallel.
92. When using a transformer algorithm in a flyback switching power supply, it always needs to be adjusted many times. Is there a more universal transformer parameter calculation method for flyback switching power supply?
Answer: Although the design of the transformer is calculated by theory, it still requires multiple tests and adjustments due to the differences in the magnetic core, winding method, etc. Generally, the primary inductance is calculated first, the magnetic core material and skeleton size are selected according to the output power, and then some parameters such as the cross-sectional area of the magnetic core are determined according to the manual. The purpose of single-ended transformer design is to reset the magnetic flux of the magnetic core.
93. Using TDS3032B and THS710 oscilloscopes, how to completely capture and store a one-time random signal, and then replay and analyze it?
A: If the so-called random signal is a single signal, then you only need to set the vertical and horizontal scales that match the signal, adjust the trigger level, use the single trigger to wait for the signal to appear, and then use SAVE/RECALL to store it in ref and call it out at any time; if the signal is an abnormality in a repetitive signal, you can first Autoset, then set
the acquisition mode to fast 500-point display, and adjust the afterglow to infinite.
94. What are the special requirements for the switching power supply to start at low temperature (such as: below -20℃)?
A: The key is the temperature range of the device selection. For example, capacitors, MOSFETs, diodes, etc.
95. The switching power supply will always have electromagnetic radiation, and it is more likely to be interfered by other electrical equipment. How can we achieve the goal of not being interfered by other electrical appliances and effectively radiating the device outward?
A: Because the switching power supply works in a high-voltage and high-current switching state, the electromagnetic compatibility problems caused by it are quite complicated. From the perspective of the electromagnetic compatibility of the whole machine, there are mainly common impedance coupling, line coupling, electric field coupling, magnetic field coupling and electromagnetic wave coupling. The three elements of electromagnetic compatibility are: interference source, propagation path and interfered object. Common impedance coupling mainly refers to the common impedance between the interference source and the interfered object, through which the interference signal enters the interfered object. Inter-line coupling mainly refers to the mutual coupling caused by parallel wiring of the conductors or PCB lines that generate interference voltage and interference current. Electric field coupling mainly refers to the coupling of the induced electric field on the interfered object due to the existence of potential difference.
Magnetic field coupling is mainly the coupling of low-frequency magnetic field generated near the pulse power line of large current to the interference object. Electromagnetic wave coupling is mainly due to the high-frequency electromagnetic wave generated by the pulsating voltage or current, which radiates outward through space and produces coupling to the corresponding interfered body. In fact, each coupling mode cannot be strictly distinguished, but the emphasis is different. From the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility of switching power supply, we can start from three aspects.
1) Reduce the interference signal generated by the interference source;
2) Cut off the propagation path of the interference signal;
3) Enhance the anti-interference ability of the interfered body.
When solving the electromagnetic compatibility inside the switching power supply, the above three methods can be used in combination,
with the cost-effectiveness ratio and the difficulty of implementation as the premise. The external interference generated by the switching power supply, such as power line harmonic current, power line conduction interference, electromagnetic field radiation interference, etc., can only be solved by reducing the interference source.
On the one hand, the design of input and output filter circuits can be enhanced, the performance of active power factor correction (APFC) circuits can be improved, the voltage and current change rate of the switch tube and the rectifier freewheeling diode can be reduced, and various soft switching circuit topologies and control methods can be adopted.
On the other hand, the shielding effect of the casing should be strengthened, the gap leakage of the casing should be improved, and good grounding should be carried out. For external anti-interference capabilities, such as surges and lightning strikes, the lightning protection capabilities of the AC input and DC output ports should be optimized. Usually, for the combined lightning waveform of 1.2/50μs open-circuit voltage and 8/20μs short-circuit current, due to its small energy, a combination of zinc oxide varistor and gas discharge tube can be used to solve it.
To reduce the internal interference of the switching power supply, realize its own electromagnetic compatibility, and improve the stability and reliability of the switching power supply, the following aspects should be taken into consideration:
pay attention to the correct distinction between the PCB wiring of digital circuits and analog circuits, and the correct decoupling of the power supply of digital circuits and analog circuits;
pay attention to the single-point grounding of digital circuits and analog circuits, and the single-point grounding of large current circuits and small current circuits, especially current and voltage sampling circuits, to reduce common impedance interference and reduce the influence of ground loops;
pay attention to the spacing between adjacent lines and signal properties when wiring to avoid crosstalk; reduce ground line impedance; reduce the area surrounded by high-voltage and high-current lines, especially the primary side of the transformer and the switch tube, and the power supply filter capacitor circuit;
reduce the area surrounded by the output rectifier circuit and the freewheeling diode circuit and the DC filter circuit; reduce the leakage inductance of the transformer and the distributed capacitance of the filter inductor; use filter capacitors with high resonant frequency, etc.
The power test solution launched by TEK can perform pre-consistency tests on current harmonics according to the EN61000-3-2 standard.
96. What data is the SOA test obtained from, and what measurement method of the oscilloscope can be used to obtain this data?
Answer: SOA is the safe operating area measurement, which is used to determine the reliability of power devices. When a short circuit or power-on occurs, the safe operating area may be exceeded for only a few cycles, and this is not easy to detect. The impact on the device will not damage it, but it is also an accumulation for the device, and the device margin may not be enough.
97. How to test the jitter component with an oscilloscope?
Answer: Deterministic jitter can be measured with an oscilloscope. The time width of the rising/falling edge can be read on the oscilloscope. According to the signal period, it can be converted into UIp-p, which is the peak amplitude of the jitter, as shown in the figure below. For more details, please refer to the relevant information of oscilloscope manufacturers such as Tektronix.
98. How to distinguish between analog bandwidth and digital real-time bandwidth?
Answer: Bandwidth is one of the most important indicators of an oscilloscope. The bandwidth of an analog oscilloscope is a fixed value, while the bandwidth of a digital oscilloscope has two types: analog bandwidth and digital real-time bandwidth. The highest bandwidth that a digital oscilloscope can achieve for repetitive signals using sequential sampling or random sampling technology is the digital real-time bandwidth of the oscilloscope. The digital real-time bandwidth is related to the highest digitization frequency and the waveform reconstruction technology factor K (digital real-time bandwidth = highest digitization rate/K), and is generally not given directly as an indicator. From the definitions of the two bandwidths, it can be seen that the analog bandwidth is only suitable for the measurement of repetitive periodic signals, while the digital real-time bandwidth is suitable for the measurement of both repetitive signals and single-shot signals. When manufacturers claim that the bandwidth of an oscilloscope can reach so many megabytes, it actually refers to the analog bandwidth, and the digital real-time bandwidth is lower than this value. For example, the bandwidth of TEK's TES520B is 500MHz, which actually means that its analog bandwidth is 500MHz, while the highest digital real-time bandwidth can only reach 400MHz, which is much lower than the analog bandwidth. Therefore, when measuring a single-shot signal, it is necessary to refer to the digital real-time bandwidth of the digital oscilloscope, otherwise it will bring unexpected errors to the measurement.
99. Can an oscilloscope be used as a digitizer?
Answer: The fastest oscilloscopes and digitizers usually use parallel flash converters and 8-bit resolution. 8 bits or 256 levels of digitization are sufficient to express a relatively smooth and easy to understand waveform display. Therefore, why not use a digital storage oscilloscope (DSO) as a digitizer, especially for high-speed signals, it is difficult for both instruments to obtain resolutions above 8 bits.
In fact, the results of doing so are satisfactory, but there are exceptions. Oscilloscopes are non-continuous acquisition instruments and digitizers are not like that. After the oscilloscope captures a signal, it needs a place to put the data before capturing more signals, unless continuous waveform acquisition similar to the frame rate of a TV is used to store the data in a pixel image. Such acquisition and equivalent display rate is very high, but the data format makes the amount of data for further external analysis very large.
In addition to the special processing mentioned above, oscilloscopes can only continuously acquire and display signals at very low speeds. Digitizers can achieve continuous throughput rates of 100MS/s or higher, limited only by the speed of the memory bus. For example, a digitizer card for the PCI bus has a data transfer rate of 100MB/s, and the PCI bus can work up to 66MS/s (132MB/s).
The throughput rate of an oscilloscope is limited by the data processing speed of the slower, low I/O capabilities. Slower digitizers and data recorders can write data directly to the hard disk and archive several GB of data, while oscilloscopes generally only have a maximum of 16MB. If you look at the data transfer rate from another perspective, many applications only need to capture sporadic data, but these bursts may be very close. At this time, it is very important to transfer data records quickly. Such signals include high repetition frequency (PRF) scanning radar, time-resolved ultrasonic sonar, time-of-flight mass spectrometers, and nuclear counting applications.
100. What is a combination oscilloscope?
Answer: A combination oscilloscope is an oscilloscope that combines the capabilities and advantages of both an analog oscilloscope and a digital storage oscilloscope (DSO). When the combination oscilloscope is set as a DSO, the user can use it to perform automatic parameters, measurements, store acquired waveforms and then make hard copies; at the same time, when needed, it can also have the infinite resolution of an analog oscilloscope and the familiar and reliable waveform display. When using the combination oscilloscope, no matter how high or low the signal repetition rate is, the brightest display can be obtained.
Keywords：Oscilloscope
Reference address：100 Questions about Oscilloscopes
Answer: Oscilloscopes have long been one of the most effective tools for testing electronic circuits. By observing the voltage and current waveforms at the key nodes of the circuit, you can visually check whether the circuit is working properly and verify whether the design is appropriate. This is extremely helpful for improving reliability. Of course, the correct analysis and judgment of the waveform depends on the engineer's own experience.
2. What are the main factors that determine the price of oscilloscope probes?
Answer: There are many types of oscilloscope probes with different performances, such as high voltage, differential, active high-speed probes, etc. The price ranges from a few hundred yuan to nearly ten thousand US dollars. The main determinants of price are of course bandwidth and function. The probe is the part of the oscilloscope that contacts the circuit. A good probe can provide the fidelity required for testing. To achieve this, even if it is a passive probe, there must be a lot of passive device compensation circuits (RC networks) inside.
3. How long is the service life of a general oscilloscope probe? Does the probe need to be calibrated regularly?
Answer: It is difficult to say the life of an oscilloscope probe. It depends on the environment and method of use.
The standard does not have clear measurement regulations for probes, but for passive probes, at least when replacing probes or exchanging channels, probe compensation adjustment must be performed. All active probes should be preheated for at least 20 minutes before use, and some active probes and current probes need zero drift adjustment.
4. What is the real-time sampling rate of an oscilloscope?
Answer: The real-time sampling rate refers to the reciprocal of the sampling interval of an oscilloscope's one acquisition (one trigger). It is understood that the highest level in the industry is the simultaneous use of four channels.
5. What is the equivalent time sampling of an oscilloscope?
Answer: Equivalent time sampling means that the oscilloscope pieces together the waveforms collected by multiple acquisitions (multiple triggers) into one waveform. The sampling rate may be very slow each time, and there is a certain offset between the two acquisition trigger points. The reciprocal of the minimum sampling interval between the two points formed in the end is called the equivalent sampling rate. Its index can reach a very high level, such as 1ps.
6. What is power factor? How to measure it?
Answer: Power factor: In a DC circuit, voltage multiplied by current is active power. But in an AC circuit, voltage multiplied by current is apparent power, and the part of power that can do work (i.e., active power) will be less than the apparent power. The ratio of active power to apparent power is called power factor, which is expressed as COSΦ. In fact, the simplest way to measure it is to measure the phase difference between voltage and current, and the result is power factor.
7. How to express and test power density?
Answer: Power density is the power per unit volume, and W/in3 is generally used in power supplies.
8. Is there any way to use an oscilloscope to measure the working conditions of a high-frequency transformer or inductor core?
Answer: There is a function in the power test solution launched by TEK - BH curve analysis, which can reflect the working state of the core, measure the dynamic inductance value, and obtain the core loss.
9. There are many types of noise in switching power supplies, such as cross interference caused by unreasonable wiring, inductor leakage, diode reverse spikes, etc. How to identify noise with an oscilloscope?
Answer: TEK's TDS5000 oscilloscope has frequency domain analysis. By analyzing the frequency band of noise, the type of noise can be analyzed, so that the corresponding processing method can be used. Oscilloscopes can only provide data analysis and band display.
10. How can an oscilloscope be used to test the radiation of the switching power supply?
Answer: There is radiation interference in the switching power supply. The general practice is to try to find the interference source and then shield it. The oscilloscope can use the Fourier transform function to analyze its frequency component composition, and determine the type of interference based on the frequency range.
11. In the design process of flyback power supply, the conversion efficiency of the transformer is often reduced due to the large leakage inductance of the transformer. When winding, the method of winding the secondary in the middle of the primary is still not ideal. Are there any skills in winding the transformer?
Answer: Wrap the high-power output winding inside, as close to the primary as possible, to strengthen the coupling.
12. Is there an oscilloscope that can analyze switching loss?
Answer: Tektronix's power supply test system, namely the TDS5000 series digital phosphor oscilloscope plus the TDSPWR2 power analysis software, can easily analyze the switching loss and the power loss per cycle, even including RDS ON.
13. Can the oscilloscope perform Fourier decomposition?
Answer: Most modern digital oscilloscopes have FFT functions, and the above system can even pre-test current harmonics according to the EN61000-3-2 standard.
14. Can the oscilloscope perform filtering? For example, low-pass filtering of PWM waves?
Answer: TDS5000 can perform 20MHz and 150MHz low-pass filtering, and can also perform a digital low-pass filtering called high-resolution acquisition. In this mode, the vertical resolution of the sampling point can be increased from 8 bits to 12 bits. The above system can output a sine wave waveform similar to that of a signal such as PWM according to the trend of pulse width change.
15. When using a digital oscilloscope, what are the principles for setting the B trigger and trigger level and the measured signal?
Answer: Tektronix's oscilloscope supports A, B trigger function. Simply put, it can trigger dual event sequence. When AB seq is selected, the A event is used as the main trigger, and the B event is used to capture complex waveforms. The trigger method is the A event arm trigger system, which triggers at the B event when the defined B event occurs. For detailed trigger instructions, please refer to the oscilloscope manual.
16. How to use TDS3052B to measure the maximum value of the modulated wave with a carrier frequency
of tens of K and a modulating wave frequency of the power supply frequency? Answer: The power frequency input may be a low frequency of 50Hz/60Hz, and the carrier frequency is tens of K. A power frequency cycle is about 20ms. If the oscilloscope needs to observe a 20ms signal, the duration acquisition window of the oscilloscope must be at least 2ms/div ×10 grids. At the same time, the sampling rate of the oscilloscope is determined based on the carrier signal of tens of K. Finally, the required acquisition memory length can be estimated to determine whether it can meet the test requirements.
17. Using a nominal 100MHz DSO oscilloscope to measure a high-frequency switch with an amplitude of 400V and f=50M, how does the oscilloscope depict its waveform and rise time?
Answer: ① The bandwidth of the oscilloscope is defined as the bandwidth of the sine wave amplitude attenuation -3dB point.
② The description of the waveform and rise time in the digital oscilloscope is obtained by obtaining waveform data through real-time sampling circuits and high-speed A/D converters, and then interpolation operations are performed.
③ In Tektronix's oscilloscope, there is a real-time processing circuit to complete the so-called sine interpolation function, which is completed in the signal acquisition circuit part. Of course, many oscilloscopes also complete mathematical operations through the main processor of the oscilloscope, which will take more time.
④ For the signal you measured, I am afraid that it is impossible to use a 100MHz oscilloscope. For a 50MHz square wave, in theory, an oscilloscope of more than 450MHz should be used to accurately reproduce the most important 9th harmonics in the signal, so as to ensure that the waveform is not distorted. What's more, you may also have to consider the signal rise time. In theory, the rise time of the oscilloscope should be more than 5 times faster than the signal.
⑤ The same is true for probes. Since ordinary probes will produce high-frequency distortion when measuring high voltages, you should use special differential probes or high-voltage probes such as Tektronix's P5205 and P5100 for measurement.
18. How to use digital oscilloscopes in analog circuits, such as measuring small signals of audio amplifiers, noise in power supplies, etc.?
Answer: The following issues need to be noted:
① The grounding of the oscilloscope. The reference ground wires of the oscilloscope housing and probes are connected to the ground wire, so good grounding is the primary condition for measuring interference.
② The interference introduced by the reference ground wire of the oscilloscope. Since ordinary probes usually have a ground wire, they will form an interference path similar to a loop antenna with the point to be measured, introducing relatively large interference. Therefore, to minimize this interference, you can remove the probe cap and do not use the ground wire led out from the probe, but directly use the probe tip and the ground inside the probe to contact the point to be measured for measurement.
③ Use differential measurement methods to eliminate common-mode noise. Tektronix provides a series of differential probes, such as the ADA400A specifically for small signals, which can measure hundreds of microvolts, and the P7350 for high-speed signal measurement provides a bandwidth of up to 5GHz.
④ Many Tektronix oscilloscopes provide a high-resolution acquisition (Hi-Res) signal capture mode, which can filter random noise superimposed on the signal.
19. When measuring the conducted disturbance of the signal line off the board, two large noise signals were found at two specific frequency points (one is 659K and the other is 1.977K). Preliminary analysis shows that it is caused by the switching power supply chip on the board. How to use an oscilloscope to measure such noise signals?
Answer: There are several factors to consider when testing noise signals with an oscilloscope:
① The amplitude of the measured signal, whether it is a small signal, the oscilloscope can test uA-level signals with the probe
② The frequency of the measured signal.
③ Improper connection of the probe will generate noise and affect the test results.
20. When using a Tektronix oscilloscope, how do you understand the Holdoff parameter?
Answer: Holdoff (trigger holdoff) means temporarily closing the trigger circuit of the oscilloscope for a period of time (i.e., holdoff time). During this period of time, the oscilloscope will not trigger even if there is a signal waveform point that meets the trigger condition. In digital oscilloscopes, it is also expressed as a percentage, which means the percentage of the entire record length or the entire screen.
The function of the trigger part of the oscilloscope is to display the waveform stably, and trigger holdoff is also a function set to display the waveform stably. It is specially designed for large-cycle repetitions where there are many non-repetitive waveform points that meet the trigger conditions. For example, as shown in the figure, the red points in the figure can all meet the trigger conditions. If the holdoff function is not used, the trigger point will not be fixed, resulting in unstable display. After using trigger holdoff, the trigger is triggered at the same point each time, so the display can be stable.
In addition, trigger holdoff is also required for amplitude modulation signals. For details, please refer to the Tektronix article "Oscilloscope XYZ".
21. Regarding holdoff, what is the difference between triggering and non-triggering, and how the oscilloscope processes the acquired signal?
Answer: For digital oscilloscopes, regardless of whether they are triggered or not, the oscilloscope is actually constantly acquiring waveforms, but only stable triggers can produce stable displays. This situation may also occur when the oscilloscope trigger circuit mode is in "automatic" mode, that is, the waveform is displayed regardless of whether the trigger conditions are met. If the "normal" mode is used, the waveform will not be displayed if the trigger conditions are not met.
22. Regarding holdoff, if the horizontal time resolution remains unchanged, does the larger the percentage setting (the corresponding signal display gradually stabilizes) mean that the signal cycle is longer?
A: Yes, the larger the percentage, the longer the holdoff time.
23. How to use an oscilloscope to measure differential signals?
A: The best way is to use a differential probe. The measured signal is the most real and objective. If there is no differential probe, you can use two differential probes to connect to the two channels of the oscilloscope (such as Ch1, Ch2), and then use mathematical operations to get the waveform of ch1-ch2 and analyze it. At this time, try to keep the two probes exactly the same, and the Vertical scale (how many volts per grid) of the two channels of the oscilloscope the same, otherwise, the error will be large.
24. How to use an oscilloscope to measure the differential signal on the USB bus?
A: There are two situations for testing USB signals:
The first is to comply with the physical layer test specifications of the USB1.1/2.0 bus defined by the USB organization. Only after passing the USB consistency test can the USB logo be marked. The USB physical layer consistency test is divided into many test items, which mainly examine the signal quality of the USB signal, such as
Signal Quality Test
Droop & Drop Test
Inrush Current Test
HS Specific Tests
Chirp Test
Monotonic Test
Receiver sensitivity Test
Impedance Test (TDR) and so on.
The second case is to observe only the signal on the USB bus. You can choose a suitable differential probe to connect to D+, D-, and directly observe the USB signal. The USB2.0 signal speed is relatively fast, with a rise time of several hundred picoseconds. In order to ensure the signal packet truth test, it is necessary to select an oscilloscope greater than 2GHz and a differential probe for testing.
25. High-speed signal characteristics on PCB board: XAUI interface 3.125GBd serial differential signal: 60ps, what bandwidth of oscilloscope is required for accurate measurement? What is the measurement error?
Answer: The 3.125GBd serial differential signal of the XAUI interface sounds a bit like an InfiniBand signal. It is collected by sinusoidal interpolation or similar equivalent sampling. However, due to factors such as bandwidth and trigger jitter, when measuring the rise time in the range of 100ps to 130ps, a 7GHz differential probe can ensure an error of <3%. For rise time measurement of < 80ps, the error will be greater than 10%. Although this is the best solution in a real-time oscilloscope, the most accurate solution for rise time measurement is Agilent's network analyzer (with physical layer analysis software) because its bandwidth can reach up to 50GHz.
26. For designs with high requirements for clock phase noise parameters, what key issues need to be considered to reduce phase noise?
Answer: There are many indicators for measuring performance in ADC and DAC devices: such as bit number, conversion speed, DC accuracy, switching performance, dynamic performance (SNR, SINAD, IMD), etc.
27. For designs with high requirements for clock phase noise parameters, how to measure phase noise?
A: From the perspective of the oscilloscope, the amplitude, time, converted signal quality, conversion speed, clock and data setup/hold time and other parameters of the analog and digital signals of the ADC and DAC can be tested. The advanced calculation function (spectrum analysis function) in the TDS oscilloscope can also be used to qualitatively measure parameters such as SNR and SINAD.
28. Since it may be necessary to introduce an external clock, there is a problem of choosing one from two. What solution can be used to minimize the deterioration of phase noise?
A: First, we need to analyze the source of jitter. Oscilloscope is a good tool for jitter analysis. Currently, we can use TDS5000B/6000B/7000B series oscilloscopes with jitter analysis software to conduct a thorough jitter analysis, such as determining jitter (Dj), random jitter (Rj), and separating Rj from Dj. Finally, we can eliminate jitter by analyzing the cause of jitter.
29. When viewing waveforms on an oscilloscope, what is the difference between using external triggering and self-triggering?
A: The usual trigger of an oscilloscope is edge triggering, which has two triggering conditions, trigger level and trigger edge; that is, when the rising edge (or falling edge) of the signal reaches a certain level (trigger level), the oscilloscope triggers. The oscilloscope will only use external triggering when there is a problem with the signal self-triggering. There is no better problem. This problem may usually be that the signal is relatively complex, there are many points that meet the trigger conditions, and it is impossible to trigger at the same position every time to obtain a stable display. At this time, an external trigger is needed. Take the following example:
Observe the signal above. Since each point ABCD will be triggered, the waveform displayed by the oscilloscope will not be stable. At this time, the following signal can be used as the trigger signal, and the oscilloscope will be able to display all cycles.
30. The bandwidth of TDS3032B is 300MHz, the sampling frequency is 2.5G/s, and the sampling frequency is 8 times the bandwidth. What is the fixed relationship between bandwidth and sampling frequency? We also have an oscilloscope from another manufacturer with a bandwidth of 100MHz and a sampling frequency of only 200MHz. Why is the bandwidth sampling frequency ratio of the two oscilloscopes so different?
Answer: Bandwidth is the most important indicator of an oscilloscope, because there is an ADC in a digital oscilloscope, and its sampling rate theoretically needs to meet the Nyquist sampling theorem, that is, each cycle of the highest frequency signal of the measured signal theoretically needs to sample at least 2 points, otherwise it will cause aliasing. But in reality, it also depends on many other factors, such as the waveform reconstruction algorithm. Tektronix oscilloscopes use advanced waveform reconstruction algorithms, and only 2.5 points are needed for each cycle of the measured signal to reconstruct the waveform. Some oscilloscopes use linear interpolation algorithms, which may require 10 points. Generally, a sampling rate of 4-5 times the bandwidth can accurately reproduce the waveform.
Tektronix's TDS3000B series is a "real-time sampling" oscilloscope, that is, its single-shot bandwidth (the ability to capture a single signal) = repetitive bandwidth. The single-shot bandwidth of the other oscilloscope you mentioned is obviously less than 100MHz. You can take a look at its indicators.
31. How to understand the bandwidth in the oscilloscope indicators?
Answer: Bandwidth is a basic indicator of an oscilloscope. It is the same as the definition of amplifier bandwidth. It is the so-called -3dB point, that is, the frequency point when the amplitude is attenuated to 70.7% of the actual amplitude when a sine wave is added to the input of the oscilloscope is called bandwidth. In other words, using an oscilloscope with a bandwidth of 100MHz to measure a 1V, 100MHz sine wave, the amplitude obtained is only 0.707V. This is only the case of a sine wave. Therefore, when we choose an oscilloscope, in order to achieve a certain measurement accuracy, we should choose a bandwidth that is 5 times the highest frequency of the signal.
32. How to obtain the total bandwidth of the measurement system?
Answer: The total bandwidth of the measurement system = 0.35/rise time (oscilloscopes below 1GHz).
33. Under the condition of a certain bandwidth, does it make little sense to have a too high sampling frequency?
Answer: Bandwidth is the basic condition to limit the capture of the high-frequency components of the measured signal. Using a Tektronix oscilloscope, only 2.5 points are needed for each measured signal cycle to reconstruct the waveform to the maximum extent. Some other oscilloscopes require more than 4 samples/cycle, that is, a 100MHZ bandwidth oscilloscope requires a sampling rate of at least 400MS/s for a single acquisition. Some oscilloscopes even require 10 points (linear interpolation technology) to ensure that the acquired signal is meaningful. [page]
34. What are the advantages and disadvantages of the so-called Gaussian response oscilloscope and flat response oscilloscope and what are their suitable fields?
Answer: There are no indicators of flat response and Gaussian response in the specifications of oscilloscopes. Similar comparisons or discussions may occur in oscilloscopes for the following reasons:
As we all know, oscilloscopes are time-domain instruments. Since Tektronix invented the first triggerable analog oscilloscope, the bandwidth of an oscilloscope has always been the most important indicator. It refers to the analog bandwidth of the preamplifier inside the oscilloscope. However, the definition of oscilloscope bandwidth is in the frequency domain, that is, the frequency point when the sine wave amplitude decays to the -3dB point. A complex high-speed signal contains a wealth of spectral components. If you need to accurately measure the signal, you must know the amplitude and phase of each of their spectral components, so the amplitude-frequency characteristic and phase-frequency characteristic of the oscilloscope are very important.
From the development in recent years, the bandwidth of digital oscilloscopes is getting higher and higher. From the launch of the TDS7000 4GHZ bandwidth oscilloscope by Tektronix in 2000, the launch of the TDS6000 6GHZ bandwidth oscilloscope in 2001, the launch of the TDS7704B 7GHZ bandwidth oscilloscope in 2003, to the recent TDS6804B 8GHZ bandwidth oscilloscope, the bandwidth has been increasing almost every year. When the bandwidth of the oscilloscope reaches several GHZ, it is increasingly difficult for the preamplifier, as an analog device, to ensure good amplitude-frequency and phase-frequency characteristics. Tektronix is the only company that has mastered this most critical technology. Some manufacturers cannot do this, so they have to use other methods to make up for the lack of bandwidth of analog devices and obtain higher bandwidth, and the frequency response curve will naturally change.
With the increasing number of various high-speed signals and faster signal rates, new requirements are put forward for real-time oscilloscopes. Some new technologies have also appeared in the digital oscilloscopes of oscilloscope manufacturers. The most significant is that the oscilloscope uses digital signal processing technology (DSP) to obtain better performance. DSP is mainly used in digital oscilloscopes, including:
enhanced bandwidth,
faster rise time
, gain and waveform calibration and improvement,
amplitude and phase improvement,
optical reference receiver normalization,
and Tektronix's third-generation oscilloscope (DPO) is the best embodiment of DSP technology. Reasonable use of DSP can improve the signal fidelity of oscilloscope testing.
However, the use of DSP technology will confuse every oscilloscope user, especially in terms of "whether bandwidth can be improved through DSP", "the bandwidth of the oscilloscope is analog bandwidth, what is the relationship with DSP technology", "is the current oscilloscope bandwidth analog bandwidth or DSP bandwidth?" "What are the negative effects of DSP technology?"
In the latest Tektronix TDS6804B 8GHZ bandwidth oscilloscope, the analog bandwidth is 7GHZ, and the bandwidth after DSP enhancement is 8GHZ. In order to ensure that every tester understands these two methods, the DSP bandwidth enhancement function can be turned on and off in TDS6804B. Tektronix tells every tester the advantages and problems brought by DSP enhanced bandwidth, helping testers understand the test results of analog bandwidth and DSP enhanced bandwidth, and better perform high-speed signal testing.
35. In addition to Gaussian response oscilloscopes and flat response oscilloscopes, are there oscilloscopes based on other responses?
Answer: The frequency response characteristics of the oscilloscope preamplifier are the most critical factor in determining the test results, which are determined by analog devices. The key lies in what method is used to obtain sufficient frequency response.
36. When I used oscilloscopes such as TDS744 and TDS745, I used passive probes (such as P6139A, bandwidth 500M). After I bought an active probe (P6237), the test results of the two were quite different from the test waveforms (especially when measuring high-frequency signals). From the probe parameters, we know that the input capacitance of the active probe is <1pF, while that of the passive probe is about 10pF. It seems that the test results of the active probe can better reflect the true situation of the signal. Since the passive probe has a great attenuation on high-frequency signals, what is the significance of the 500M bandwidth? How to choose to use an active or passive probe according to the test situation?
Answer: The typical bandwidth value of your P6139A probe plus Tektronix's 500MHz oscilloscope can still reach 500MHz, but as you said, its input capacitance is different. This capacitance will produce a load effect on the signal to be tested, causing signal ringing and shape changes. Therefore, using an active probe at this time can reflect the true situation of the signal. In fact, when using a probe, we should not only consider bandwidth, but all these factors should be considered when measuring high-frequency signals:
bandwidth/rise time
dynamic range
load effect
ground effect
resonance effect
Especially when using P6139A, you should also consider the influence of the ground wire. The ground wire on the probe will also cause ringing. When measuring high-frequency signals, the length of the ground wire should be shortened as much as possible.
In addition, the P6247 you use is an active differential probe, and the influence of common mode may also be a factor.
The main reason for choosing a passive probe is its large dynamic range. For example, P6139A can measure signals from millivolts to hundreds of volts, while P6247 can only measure +-8.5V signals. In addition, the price of active probes is also a factor.
37. During the experiment, the MOSfet exploded after the oscilloscope was grounded. Now the ground wire of the oscilloscope has been cut off. What is the reason?
Answer: In order to ensure personal safety during the test and obtain good measurement results, the ground wires of all probes of the oscilloscope are generally connected to the casing and connected to the ground wire of the oscilloscope power cord. Therefore, when you measure the waveform of the MOSFET tube in the power supply, if any point is not ground, there will be problems, as shown in the figure below.
Cutting the ground wire can prevent short circuit problems in the MOSFET tube test, but it will also bring some other test problems, such as the oscilloscope case being charged, and the oscilloscope case distribution parameters affecting the measured signal. The solution is to use a differential probe, such as Tektronix's P5205, which can measure the differential signal when neither of the two test points is ground.
38. When using an oscilloscope to capture data, it is found that the stored text only contains the data on the current screen, and the time interval is based on resolution. How to use software to process data in real time (matlab?), how to capture more data?
Answer: Tektronix oscilloscopes use a compressed screen display style, that is, the waveform displayed on the screen is all the collected data. With the multiViewZoom function of TDS5000B, all waveforms can be easily displayed.
Tektronix TDS5000B, TDS6000, TDS7000B, TDS8000B series oscilloscopes all use a completely open WINDOWS platform and support all current popular tools, such as Matlab, LabView, VB, VC, .NET, MicroSoft Office VBA, etc., which can flexibly perform data analysis and processing.
These analysis tools can also be directly installed in the oscilloscope to form an instrument that integrates data acquisition, analysis, display, and processing. To collect more data at a time, the oscilloscope needs to be equipped with a deeper storage depth, such as the TDS5000B series general-purpose oscilloscope, which can support up to 16M memory.
39. What are the factors that affect the working speed of the oscilloscope?
Answer: In fact, the principles of any oscilloscope are similar. The front end is a data acquisition system and the back end is computer processing. There are two main aspects that affect the speed. One is the data transmission from the front-end data acquisition to the back-end processing, which generally uses the PCI bus. This is a transmission bottleneck, but there are new technologies that can break through it; the other is the back-end processing method. Improving the processing speed can be achieved through data packet sharing.
40. Our applications usually capture 2M or even more data for analysis, and the sampling rate is usually as high as 10GS/S, but it always seems very slow when performing parameter testing and FFT analysis. Why?
Answer: The processing speed is naturally slow when the amount of data is large. In order to obtain high-speed real-time FFT analysis of large amounts of data, a dedicated FFT processor must be used, but the cost is high.
41. When using Tektronix's TDS2014 digital oscilloscope to capture the timing of a parallel port, a very strong 50Hz AC can always be measured, but no signal can be measured. However, the ground of the oscilloscope is consistent with the ground of the measured parallel port. What should I do?
Answer: You can start from the following aspects:
① Check whether the oscilloscope is well grounded or isolated by an isolation transformer;
② Whether there is a strong 50Hz signal induction nearby;
③ In a strong interference environment, you should pay attention to whether the parallel port's driving capability and operating frequency are suitable for the test operation selection. If you only see a 50Hz interfering sine wave, and the waveform is relatively regular, you should consider that the parallel port may not be working;
④ Check if the probe tip is damaged;
⑤ It is recommended to unplug all unnecessary peripherals, which may also come from the monitor;
⑥ If the oscilloscope has been used for a long time, you should consider whether the bottom line is normal, that is, the small clip. Remove the probe and measure it with a multimeter.
42. To solve the problem of power supply interference, you want to measure the situation where the interference signal of the total power supply is connected to the power supply of the weak signal amplifier. As a result, even if the oscilloscope probe is connected to the ground, there is an interference signal, no matter where it is measured. The interference signal is audio. Why is this?
Answer: The following issues need attention:
① The grounding problem of the oscilloscope. The reference ground wires of the oscilloscope housing and the probe are connected to the ground wire. Therefore, good grounding is the primary condition for measuring interference.
② The interference introduced by the reference ground wire of the oscilloscope. Since ordinary probes usually have a ground wire, they will form an interference path similar to a loop antenna with the point to be measured, introducing relatively large interference. Therefore, to minimize this interference, you can remove the probe cap and do not use the ground wire led out from the probe. Instead, directly use the probe tip and the ground inside the probe to contact the point to be measured for measurement.
③ Use differential measurement methods to eliminate common-mode noise. Tektronix provides a series of differential probes, such as the ADA400A specifically for small signals, which can measure hundreds of microvolts, and the P7350 for high-speed signal measurement provides a bandwidth of up to 5GHz.
④ Many Tektronix oscilloscopes provide a high-resolution acquisition (Hi-Res) signal capture mode, which can filter out random noise superimposed on the signal.
43. Sometimes in EMC test, the indicator disappears for a short time. When using an oscilloscope for testing, it is found that the entire screen of the oscilloscope shakes during the test. The test item is EFT (transient pulse train immunity test). How to explain and eliminate this phenomenon in the test?
Answer: EFT sometimes interferes with the oscilloscope and causes false triggering. You can try to use the high-frequency suppression trigger mode of the oscilloscope, limit the bandwidth of the oscilloscope, etc.
44. Why can't the oscilloscope sometimes capture the amplified current signal?
A: If the signal does exist, but the oscilloscope can sometimes catch it and sometimes not, this may be related to the settings of the oscilloscope. Usually, if you can set the oscilloscope trigger mode to Normal, set the trigger condition to edge trigger, and adjust the trigger level to an appropriate value, and then set the scan mode to single mode, if this method still does not work, there may be something wrong with the instrument.
45. How can the new digital oscilloscope be used for single-chip microcomputer development?
A: In the process of single-chip microcomputer circuit development, generally speaking, there are no problems with the components and chips themselves. The problem is often that the communication between them is different from what is expected. In single-chip microcomputers, common buses are SPI, I2C, USB, LIN, CAN, 54621A and 54621D oscilloscopes themselves support the trigger function of serial signals, and can directly debug the communication on the serial bus. In addition, if you use DSP combined with MCU to develop a circuit board, it may involve software and hardware joint debugging. At this time, you can use the digital logic channel of 54621D to connect to the control line or data and address line to determine whether the circuit can work normally under specific operating conditions or subroutine operation. Moreover, its 2M-point storage depth per channel is very helpful in analyzing the cause of the problem, observing the serial signal for a long time, observing the handshake timing, etc. And its amplification function can amplify the signal by tens of thousands of times to observe the details.
The price of 54621A should be around US$2500, and the price of 54621D should be less than US$4000. You can visit http://www.agilent.com/find/mso http://www.agilent.com/find/test to download many related application articles.
46. Do the new digital oscilloscopes 54621A and 54621D have any effect on the different signals and different rates of the (Inter-IC) bus during testing?
Answer: The I2C Bus signal generally works at a rate of no more than 400Kbit/s. Recently, chips with several Mbit/s have also appeared. When setting the trigger conditions for 54621A and 54621D, you do not need to consider the impact of different rates. However, for other buses, such as the CAN bus, you must first set the actual current working rate of the CAN bus on the oscilloscope so that the oscilloscope can correctly decode the protocol and trigger correctly.
47. In addition to the oscilloscopes 54621A and 54621D, what other instruments can detect and analyze Inter-IC bus signals?
Answer: If you want to conduct further analysis on the Inter-IC bus signal, such as protocol-level analysis, you can use Agilent's logic analyzer, but relatively speaking, the price is higher than that of 54621A/D.
48. What kind of signals are suitable for testing various trigger applications of digital oscilloscopes, such as edge trigger, glitch trigger and pulse width trigger?
Answer: ① Edge trigger, edge trigger, you can set the trigger level, rising edge or falling edge. Edge trigger is also called basic trigger.
② Advanced trigger, which includes various trigger functions. You can set the corresponding trigger conditions according to the characteristics of the measured signal and locate the waveform of interest.
Advanced trigger is the key to circuit debugging. During the circuit debugging process, if you do not know the possible problems of the measured signal in advance, you can first use the Tektronix digital phosphor oscilloscope to use the 400,000/second waveform capture speed to quickly find various problems in the circuit, and then use different advanced trigger functions to locate the details of the fault, which can shorten your debugging cycle.
49. Regarding glitch measurement, I have consulted relevant technicians before, and the answer I got was that the minimum glitch that an oscilloscope can capture is the sampling rate of the oscilloscope. Do all oscilloscopes follow this rule? Will the pre-filter of the oscilloscope have no effect on it at this time?
Answer: It cannot be asserted that all oscilloscopes are like this. For example, some oscilloscopes reach 1GS/s, and the bandwidth is only 60MHz. Obviously, it is impossible to capture a 1ns glitch. In fact, the ability to capture glitches depends not only on bandwidth and sampling rate, but also on waveform capture rate, that is, the number of waveforms that can be captured per second. For details, please refer to Tektronix's application article on DPO.
50. How to eliminate glitches when using an oscilloscope?
Answer: If the glitches are inherent in the signal itself, and you want to use edge triggering to synchronize the signal (such as a sine signal), you can use the high-frequency suppression trigger method, which can usually synchronize the signal. If the signal itself has glitches, but you want the oscilloscope to eliminate the glitches and not display the glitches, it is usually difficult to do so. You can try to use the bandwidth limitation method, but you may accidentally filter out part of the information of the signal itself. If you use a logic analyzer, generally speaking, using the state acquisition method, some glitches collected in the timing mode will not be visible.
51. In actual work, when encountering sudden glitch signals, how to capture and test them?
A: For example, when we are testing the clock, we often encounter occasional glitch signals, which will cause malfunctions in our circuits. Therefore, capturing the signal becomes the key to the test. Since we cannot determine whether the glitch is positive or negative in advance, we must first use the digital phosphor function of the TDS5000 oscilloscope, that is, the fast waveform capture mode combined with infinite afterglow to view the glitch characteristics, and then use the advanced trigger function of the oscilloscope - pulse width triggering according to the signal characteristics, such as: triggering when the pulse width is less than the normal clock pulse width.
52. What are the applications of glitch/pulse width triggering?
A: There are generally two typical applications for glitch/pulse width triggering. One is to synchronize circuit behavior, such as using it to synchronize serial signals, or for applications with very serious interference, it is impossible to use edge triggering to correctly synchronize signals. Pulse width triggering is an option; the other is to find abnormal phenomena in the signal, such as narrow glitches caused by interference or competition. Since the abnormality appears occasionally, it must be captured by glitch triggering (another method is peak detection, but the peak detection method may be limited by its maximum sampling rate. At the same time, it is generally visible but not measured). If the pulse width of the object under test is 50ns, and there is no problem with the signal, that is, there is no signal distortion or narrower due to interference, competition, etc., the edge trigger can be used to synchronize the signal without using glitch trigger. Many users set the pulse width trigger to 10ns ~ 30ns. Fortunately, 5462x and 5464x are rare instruments in the industry that can complete this operation. If you want to verify whether there are abnormal pulses in the 10MHz square wave, including pulses much narrower than 50ns, you will use pulse width or glitch trigger, and you may use the 5ns setting.
53. Does Agilent's digital oscilloscope have DPO function?
Answer: DPO is a special term. Only one oscilloscope company uses this term. Agilent's corresponding function is called MegaVision. The similarities with DPO are: ① It can directly detect abnormal phenomena in the signal. ② The waveform capture rate is much higher than that of ordinary digital storage oscilloscopes. Differences: ① After discovering an abnormal signal, MegaVision can directly amplify the abnormality and observe the signal details. ②The real-time sampling rate of MegaVision oscilloscopes exceeds the limit of 1.25GSa/s and can reach 2GSa/s (such as 5464xA/D oscilloscopes) or even higher. ③MegaVision oscilloscopes are optimized for applications that require deep storage. When the oscilloscope storage depth is >10K, or even 100K, 2M, its waveform refresh rate is the industry's leading.
54. If the bandwidth is determined based on the signal rise time, is the principle of determining the sampling rate based on the bandwidth just to achieve no sampling aliasing error?
Answer: Determine the sampling rate after determining the bandwidth. Some formulas in the industry do determine the principle of the sampling rate in order to achieve no sampling aliasing error, but it is a general evaluation statement. It depends on the characteristics of your test object, because the highest indicators are often given under specific conditions and may not meet your test application.
55. How does an oscilloscope display the waveform between two sampling points?
Answer: There are many display modes for oscilloscopes: point display, sine interpolation display, and straight line connection display; the default display mode of an oscilloscope is usually vector connection display, and some oscilloscopes only support straight line connection; whether it is straight line connection or sine interpolation, the information provided between two actual sampling points is not actually collected, because the straight line connection may cause a sudden change in the display, such as collecting a point at the peak of a sine wave and collecting a point at the trough on both sides, a triangle wave will be displayed, while the sine interpolation display is still a sine wave. Therefore, some application articles say that: using straight line connection, the sampling rate requirement is higher, such as 10 times (to truly reproduce the waveform); using sine interpolation, the sampling rate requirement is slightly lower, and some articles say that 2.5 times is enough. In engineering, it is generally said to be more than 4 times, and there are also 5 times and 6 times.
56. High-speed signal characteristics on PCB board: 156.25MHZ differential clock signal, Rise/Fall Time (20%～80%) <100ps, jitter tolerance (pp<30ps, RMS<2ps), skew (+ vs.-) <20ps, what bandwidth oscilloscope is needed for accurate measurement? What is the measurement error?
Answer: For 156.25MHz differential clock signal, Rise/Fall Time (20%～80%) <100ps, if you want to accurately test the rise time, such as 3% test accuracy, 0.4/100ps *1.4 = 5.6GHz bandwidth oscilloscope and its probe system, if 10% accuracy is acceptable, 0.4/100ps*1.2 = 4.8GHz bandwidth oscilloscope and its probe system. Note that if you use a differential probe, you must ensure that the bandwidth of the entire oscilloscope is 5.6GHz from the point being measured. Fortunately, Agilent has launched a differential probe with a bandwidth of 7GHz. At the same time, the actual rise time indicator of the 54855A itself is 65ps, and the manual gives an indicator of 72ps. jitter tolerance (pp<30ps,RMS<2ps), to accurately measure jitter indicators, the jitter indicators of the oscilloscope itself must be higher. The trigger jitter indicator of 54855A itself is 1ps RMS, which is 7 times better than similar products in the industry. Another related indicator is Delta Time meas. Accuracy (peak) is ± [ (7.0 ps) + (1 x ppm * |reading|) ], which is more than 2 times better than similar products. This is related to the fact that it actually uses a 20GSa/s A/D, which eliminates the error caused by using multiple (10GSa/s A/D or 5GSa/s A/D) to piece together a 20GSa/s.
57. When choosing an oscilloscope, the bandwidth is generally considered. So, under what circumstances should the sampling rate be considered?
Answer: It depends on the object being measured. Under the premise of meeting the bandwidth, it is hoped that the minimum sampling interval (the inverse of the sampling rate) can capture the signal details you need. There are some empirical formulas for sampling rate in the industry, but they are basically derived for the bandwidth of the oscilloscope. In practical applications, it is best not to use an oscilloscope to measure signals of the same frequency. If you are selecting a model, for a sine wave, choose an oscilloscope with a bandwidth that is 3 times the frequency of the sine signal being measured. If the bandwidth is above 3 times, the sampling rate is 4 to 5 times the bandwidth, which is actually 12 to 15 times the signal. For other waveforms, ensure that the sampling rate is sufficient to capture signal details. If you are using an oscilloscope, you can verify whether the sampling rate is sufficient by the following methods:
stop the waveform and enlarge the waveform. If you find that the waveform has changed (such as certain amplitudes), the sampling rate is not enough, otherwise it will not be a problem. You can also use point display to analyze whether the sampling rate is sufficient.
58. A 100MHz analog oscilloscope can see the parasitic waveform more clearly, but a 100MHz digital oscilloscope cannot (it can only see the waveform in bold)?
Answer: This phenomenon is related to the oscilloscope display. The traces seen on an analog oscilloscope are generally thinner. It directly hits the voltage on the screen through a vertical deflector, and the scanning rate and waveform refresh rate are very fast. Digital oscilloscopes quantify waveform voltages through A/D, store them in memory, and display them after processing. The display resolution of digital oscilloscope screens is limited, usually 640 points or 1000 points. If you set the storage depth (record length) of the oscilloscope to 10K or 2M, this means that the information of 10K or 2M points in memory must be reflected through 640 points or 1000 points. No matter how good the algorithm is, it will bring certain display errors. The degree of waveform bolding is related to the storage depth. These problems are unique to digital oscilloscopes. In addition, the default display mode of digital oscilloscopes is vector display mode, which means that some points will be inserted between two sampling points using linear algorithms or sine interpolation algorithms. Analog oscilloscopes do not have these problems. You can try to change the oscilloscope record length to 500 points, and change the vector display to point display, observe the actual data obtained by the digital oscilloscope each sampling, adjust the time base, and you can clearly see these points. Even if you use vector display, the line will become thinner. From the perspective of the instrument, when measuring small signals, the results of using a 1:1 probe may be better than those of a 10:1 probe. In addition, analog oscilloscopes do not have the concept of sampling rate, but only the concept of scanning rate. When using a digital oscilloscope, the sampling rate often needs to be considered.
59. Which one is more advantageous when observing the details of the waveform (for example, observing parasitic waveforms below 1% when crossing the zero point and the peak)?
Answer: When observing parasitic waveforms below 1%, whether it is an analog oscilloscope or a digital oscilloscope, the observation accuracy is not very good. The vertical accuracy of an analog oscilloscope may not be higher than that of a digital oscilloscope. For example, the vertical accuracy of an analog oscilloscope with a bandwidth of 500MHz is +/-3%, which is not more advantageous than that of a digital oscilloscope (usually 1~2% accuracy). In addition, the automatic measurement function of a digital oscilloscope is more accurate than the manual measurement of an analog oscilloscope for details.
60. Digital oscilloscopes generally provide online display of the RMS value. What is its accuracy?
A: Many people use the number of A/D bits to measure the amplitude measurement accuracy of the oscilloscope. In fact, it will change with the bandwidth of the oscilloscope you use, the actual sampling rate setting, etc. If the bandwidth is not enough, the amplitude measurement error itself will be very large. If the bandwidth is sufficient and the sampling setting is very high, the actual amplitude measurement accuracy will not be as good as the accuracy when the sampling rate is low (you can sometimes refer to the user manual of the oscilloscope, which may give the actual effective number of A/D bits of the oscilloscope at different sampling rates); in general, the accuracy of oscilloscopes in measuring amplitudes, including RMS values, is often not as good as that of multimeters. Similarly, when measuring frequencies, it is not as good as frequency counters.
61. How to capture and reproduce instantaneous signals that disappear in a flash?
Answer: Set the oscilloscope to single acquisition mode (set the trigger mode to Normal, set the trigger condition to edge trigger, adjust the trigger level to an appropriate value, and then set the scan mode to single mode). If you are using Agilent 5462xA/D, 5464xA/D, 5483xB/D, 5485xA, these instruments all support the MegaZoom function, that is, you can observe the overall signal while zooming in on the local details, either by moving the screen or by using the dual time base display function. Note that the oscilloscope's memory depth will determine the time that can be collected and the maximum sampling rate that can be used.
62. Which Agilent oscilloscope can test a carrier signal with a frequency of 500M?
Answer: If you only measure the carrier signal itself, usually the carrier signal is a sine wave, it is recommended to use a 1.5GHz oscilloscope (Agilent 54845B), use a BNC cable to connect the object to be measured, and you can get a rise time measurement accuracy of ~94.6%. If a probe must be used, the 1157A active probe (2.5 GHz bandwidth) is recommended. If a 500 MHz bandwidth oscilloscope is used, even with a BNC cable, the best-case amplitude measurement error is 29.3%, and the rise time measurement accuracy is 58.6%.
63. If the oscilloscope is rated at 60MHZ, can it be understood that it can measure up to 60MHZ?
Answer: A 60MHz bandwidth oscilloscope does not mean that it can measure 60MHz signals well. According to the definition of oscilloscope bandwidth, if a 60MHz sine wave with a peak-to-peak value of 1V is input to an oscilloscope with a 60MHz bandwidth, a 0.707V signal will be seen on the oscilloscope (30% amplitude measurement error).
64. Why can't a 4.1943MHZ square wave be measured with an oscilloscope rated at 60MHZ?
Answer: If the square wave is to be tested, the reference standard for selecting the oscilloscope is the rise time of the signal. If the oscilloscope bandwidth = 0.35/signal rise time * 3, the rise time measurement error is about 5.4%.
The probe bandwidth of the oscilloscope is also very important. If the bandwidth of the system composed of the oscilloscope probe and its front-end accessories is very low, the bandwidth of the oscilloscope will be greatly reduced. If you use a 20MHz bandwidth probe, the maximum bandwidth that can be achieved is 20MHz. If you use a connecting wire at the front end of the probe, the performance of the probe will be further reduced (but it should not have much impact on the ~4MHz square wave because the speed is not very fast).
In addition, check the oscilloscope manual. Some manufacturers' newly launched oscilloscopes will have their actual bandwidth sharply reduced to <=6MHz under the 1:1 setting. For a ~4MHz square wave, its third harmonic is 12MHz and its fifth harmonic is 20MHz. If the bandwidth is reduced to 6MHz, the signal amplitude will be greatly attenuated. Even if you can see the signal, it is definitely not a square wave, but a sine wave with attenuated amplitude. Of course, there may be many reasons for not being able to measure the signal, such as poor probe contact, but this phenomenon can be easily eliminated. It is recommended to connect a function generator with a BNC cable to check whether there is any problem with the oscilloscope itself and the probe. If there is any problem, you can contact the manufacturer directly.
65. How to measure the stability of the clock?
A: If you are using 5483xB/D, 548xxA, 5484xB or 5485xA, you can use the standard configuration histogram method to measure the jitter of the clock edge or amplitude. For details, please refer to Agilent's application article: "Jitter Analysis Techniques Using an Agilent Infiniium Oscilloscope" (P/N: 5988-6109EN), which can measure the jitter in the worst case. For 5485xA, if you want more powerful jitter analysis functions, it is equipped with special jitter analysis software, which provides very powerful jitter analysis. For details, please refer to the Datasheet of the 5485x oscilloscope. For more detailed information, please call Agilent.
66. What are the methods and techniques for accurately measuring period jitter in PLL using Agilent oscilloscopes?
A: If you are using 5483xB/D, 548xxA, 5484xB and 5485xA, you can use the standard configuration of the histogram method to measure the jitter of the clock edge or amplitude. For details, please refer to Agilent's application article "Jitter Analysis Techniques Using an Agilent Infiniium Oscilloscope" (P/N: 5988-6109EN), which can measure the jitter in the worst case. For 5485xA, if you want a more powerful jitter analysis function, it is equipped with a special jitter analysis software, which provides very powerful jitter analysis. For details, please refer to http://www.agilent.com/find/test 5485x oscilloscope datasheet. For more detailed information, please call Agilent. Reminder: When using an oscilloscope, pay attention to whether its jitter-related indicators meet the test requirements, such as the trigger jitter indicators of the oscilloscope itself, etc. At the same time, pay attention to the use of different probes and probe connection accessories. If the system bandwidth of the oscilloscope cannot be guaranteed, the measurement results will be inaccurate.
67. How to use an Agilent oscilloscope to measure the Settle time of a PLL?
Answer: You can use an Agilent 548xx series oscilloscope + USB-GPIB 82357A adapter + software options to complete it. You can also use Agilent's lower-priced modulation domain analyzer to complete it.
68. Design a PLL, how to measure the dead zone of the PFD (phase detector)?
Answer: You can connect one channel of the oscilloscope to the reference signal and the other channel to the feedback signal, and set the trigger condition of the oscilloscope to the setup and hold time trigger. At this time, while adjusting the setup and hold time setting of the oscilloscope, adjust the reference signal until it loses lock. At this time, the setup and hold time setting corresponds to your PFD dead zone. Theoretically, it is believed that the loss of lock will occur at two moments: one is at the initial working time, when the phase difference (frequency difference) of the two signals exceeds the capture bandwidth of the PLL; the other is during the tracking process, when the feedback signal changes too much, causing the phase difference of the two signals to exceed the tracking bandwidth of the PLL and the lock will be lost. All Agilent 548xx series oscilloscopes can complete this measurement (under the premise that the bandwidth is sufficient).
69. How to test optical signals using Agilent equipment?
Answer: Agilent has a full set of test solutions to measure optical signals, from light sources, spectrometers, optical multimeters, optical oscilloscopes, optical wavelength meters, etc. If you want to use a real-time oscilloscope to measure optical signals, you can use an optoelectronic converter combined with an oscilloscope to complete the measurement.
70. How to use an oscilloscope to measure power supply ripple?
Answer: You can first use an oscilloscope to capture the entire waveform, then zoom in on the ripple part of interest to observe and measure (automatic measurement or cursor measurement), and also use the oscilloscope's FFT function to analyze from the frequency domain. [page]
Usually, if the details of the object being measured (amplitude, frequency, etc.) are not clear, you can use the "AutoScale" button to observe the general signal, and then adjust the horizontal control knob and the vertical control knob to get the best display (for example, the amplitude should be displayed as full screen as possible), and then use the Zoom function to fully enlarge the waveform. When measuring power supply ripple, you can use the Zoom function to enlarge the ripple part for analysis; in addition, you may consider analyzing the power supply from the frequency domain perspective to observe its harmonics and noise. To this end, you can let the oscilloscope display as many cycles of signals as possible, use the oscilloscope's storage depth as much as possible, and set the sampling rate to an appropriate value to ensure that the waveform is not distorted. The frequency resolution obtained in this way is the sampling rate divided by the current storage depth setting, and observe the amplitude difference between each harmonic and the fundamental wave. In addition, if you use MatLab software, you can use the powerful functions of MatLab software to conduct a more in-depth analysis of the captured signal data. 546xx and 548xx are both equipped with software that connects to a computer as standard, which allows data to be directly transferred to the computer for further analysis. Of course, Matlab software can also be directly installed in the 548xx.
If the circuit parameters are already known, the oscilloscope settings can be directly adjusted to allow it to work at a suitable sampling rate and vertical scale.
71. The ripple of the output voltage of a switching power supply is an important indicator. How to correctly use an oscilloscope to measure this indicator?
Answer: The definition of ripple is a clutter signal containing periodic and random components attached to the DC level, which is called PARD (Periodic And Random Deviation) in English. It is defined as the peak-to-peak value of the clutter. Things to note when measuring ripple: The ground wire of the oscilloscope probe will cause a lot of ripple. The ground wire should be unplugged and the probe internal wire should be used for measurement. Of course, the best measurement method is to use a 50 ohm terminal resistor and connect it directly to the oscilloscope with a BNC cable. It should be noted that the 50 ohm resistor should take power consumption into consideration, and a high-power resistor may be required. Relevant standard requirements, such as whether to separate periodic power frequency ripple and switching ripple, high-frequency noise, etc. Another example is whether the measurement frequency should be limited to below 20MHz.
72. When measuring ripple, a large part of it is a periodic spike pulse of 50 Hz. The larger the load current, the larger the pulse amplitude. What are the specific solutions?
Answer: In the Tektronix power measurement system, when measuring ripple, we can choose the power frequency ripple test or the switching ripple test, so that the ripple of irrelevant frequency can be automatically filtered out. For example, if you choose to test the 200KHz ripple, the oscilloscope will automatically filter other frequency components.
73. When measuring ripple, how to remove the noise on the ripple, such as power frequency noise?
Answer: The noise on the ripple can be removed by the high-resolution capture mode of the TDS5000 oscilloscope in the capture mode. These random noises. There are two types of ripples: one is the power frequency, 100HZ, and the other is the switching ripple. The TDSPWR2 launched by TEK can separate these two ripples and measure them separately to get the results.
74. When accurately testing the ripple and noise of the switching power supply, do you have to go to a special laboratory?
Answer: Of course, it would be ideal if there is a special laboratory for ripple measurement. When this condition is not met, the following issues should be noted:
① The oscilloscope should have a good grounding;
② If the measurement standard has bandwidth limit requirements, the 20MHz bandwidth limit in TDS430A should be turned on;
③ Use the AC coupling of the oscilloscope;
④ Use BNC cables and use the 50 ohm input impedance range of TDS430A for measurement (a 50 ohm high-power load, BNC adapter or test fixture may be required at this time)
To improve the measurement accuracy, the oscilloscope probe should not be used, and the ground wire of the oscilloscope probe will introduce relatively large noise.
75. How to use an oscilloscope to measure the output ripple value of some low-ripple power supplies? For example, when measuring the output ripple of 1.8V, the nominal output ripple is generally less than 20mV. How to use an oscilloscope to verify it? Even if the ordinary oscilloscope probe is directly connected to the probe ground clip, the noise is 20 to 30 millivolts.
Answer: This problem is very representative. A voltage differential probe with a high common-mode rejection ratio should be used, which can work in a high-noise environment.
76. How to use a digital oscilloscope to view and read the cycle of the displayed waveform?
Answer: All digital oscilloscopes support waveform cycle measurement. From the perspective of improving test accuracy, if you are using 5462x/5464x (except 54645), you can select Counter in its measurement parameters, and its embedded hardware frequency counter will be activated for accurate frequency measurement (5 digit). If you are using other types of oscilloscopes, try to let the oscilloscope screen display a cycle of the signal, and the amplitude should be as full scale as possible. At this time, the measurement accuracy is generally good. You can use the oscilloscope's automatic measurement function or use the cursor to measure manually.
77. I encountered a problem during development. I added functions to the prototype machine to detect the prototype's audio frequency, data output, trigger signal, etc. The test results are almost the same as the design results. Why is the prototype sound clear and accurate, while the sound of the finished product is sometimes acceptable, but sometimes not?
Answer: The sound of the actual object being tested is sometimes acceptable and sometimes not, but there is no problem with the waveform display on the oscilloscope, or the data displayed by the oscilloscope is far different from the data on the object being tested. This is often caused by the oscilloscope and your object being measured not being synchronized. You can try the following methods:
Sound signals are usually low-speed signals. You can let the oscilloscope work in scrolling mode. When observing the signal, manually stop waveform acquisition and analyze it.
Observing sound signals in the time domain is often not comprehensive. Agilent's dynamic signal analyzer is a better choice in many cases, but if you don't have this instrument, you can combine the oscilloscope's FFT function to observe from the frequency domain.
Try using the oscilloscope's trigger function. If you have a mixed signal oscilloscope (54xxxD), you can define trigger conditions in combination with its logic channels (such as state triggers and sequential triggers similar to logic analyzers).
78. How to perform clock jitter testing with a tds3012 oscilloscope?
Answer: Tektronix's open platform oscilloscopes (such as TDS7000, TDS5000) have special jitter measurement software that can perform comprehensive jitter measurements (such as Rj, Dj, etc.). In TDS3012, you can only perform relatively long-term cumulative measurements on signals through infinite persistence. In addition, jitter measurement is only required for clocks with relatively high frequencies. The general principle of measuring signals with an oscilloscope is that the bandwidth of the oscilloscope should be 5 times the highest frequency of the signal. If the square wave with a relatively fast rise time may require an oscilloscope bandwidth that is 10 times or even higher than the signal frequency. Therefore, it is recommended to use an oscilloscope with a higher bandwidth and development platform.
79. How to use an oscilloscope to measure the power factor in an AC/DC switching power supply?
Answer: In fact, using an oscilloscope to measure the power factor is to measure the phase difference between voltage and current, that is, cosφ. At the same time, the Tektronix TDS5000 power test system also automatically measures the relevant parameters of PFC (such as THD, True Power, Apparent Power, Power Factor, etc.).
80. The FFT function of the Tektronix oscilloscope can see the frequency and amplitude of the radiation of the switching power supply, but is the amplitude value here the same as the value of the certification center? If not, how to convert it? Moreover, if you select different V/DIV when viewing the waveform, there will be different amplitudes in the FFT state. Is this normal? ---The model I use is TDS1012.
Answer: The amplitude measured by the FFT function of the oscilloscope can only be used as a qualitative analysis, not a quantitative analysis, so it is only of reference value. If you want to analyze the spectrum amplitude, you can choose the Blackman-Harris window, which will have a better effect; when converting V/div, it will definitely affect the amplitude of the FFT, because this is limited by the resolution of the ADC of the oscilloscope itself. Therefore, in order to improve the measurement accuracy, it is generally chosen to fill the entire screen with the waveform as much as possible (but never exceed the screen), that is, to choose a smaller V/div gear.
81. What type of oscilloscope can effectively improve design efficiency?
Answer: At the current stage of oscilloscope development, data analysis has been raised to an important position. Using an oscilloscope is not only to observe waveforms during debugging, but more importantly, it can analyze and calculate device parameters in design to help everyone optimize the design plan. What kind of oscilloscope is most suitable should be determined in combination with the signal you want to observe and analyze.
82. How to use an oscilloscope to test video parameters (including video output level, horizontal resolution, brightness amplitude-frequency response, chroma amplitude-frequency response, brightness signal-to-noise ratio, chroma signal-to-noise ratio, brightness nonlinear distortion and other video parameters)?
Answer: Tektronix TDS3000B series oscilloscopes plus TDS3VID or TDS3SDI and TDS5000 series oscilloscopes all provide powerful video measurement functions, even including analog HDTV functions, and built-in vector oscilloscope capabilities to help you analyze various video parameters.
83. At the high frequency end, how to determine the impact of the impedance of the oscilloscope probe itself on the signal?
Answer: The probes of the oscilloscope have specific indicators. You can refer to the equivalent impedance-frequency diagram of the probe to determine the equivalent impedance of the probe at the frequency point. Regarding probes, Tektronix has a special article called "Probe ABC".
84. Why is the waveform ringing of a 30MHz clock tested by a Tektronix oscilloscope much larger than that of an Angelen (the oscilloscope probe is 250MHz)?
A: When measuring state transition, you only need to use the automatic trigger mode of the oscilloscope to set the voltage and current waveforms to a more ideal display mode. If you use TDS5000, you can also adjust the resolution knob to set the sampling rate to a suitable level (generally about 10 times the signal frequency). Then use PWR2 software to automatically calculate the measured data. For MOSFET, we choose Vds and Ids as the measured signals, and for IGBT, we choose Vce and Ice as the measured signals.
When using a digital oscilloscope to test a switching power supply, can you pre-set limit parameters (such as test time, number of samples per time)? How to use a Tektronix oscilloscope to test the state transition of a switching power supply. Connection method (with examples), oscilloscope button settings, and necessary precautions.
85. When designing a soft-switching PWM converter (such as a PWM half-bridge switching converter), how do you use an oscilloscope to observe the MOSFET Vt/It trajectory?
A: First, the oscilloscope must have a time delay correction function between channels, so that basic accuracy can be guaranteed when performing related mathematical operations. Use a high-voltage differential voltage probe and a current probe for measurement. The power test solution launched by TEK can dynamically observe the entire working process of MOSFET.
86. The selection of output capacitor and output inductor should be determined according to the power supply demand of the load. Should the L and C values be applied according to the formula determined on the datasheet? If the value calculated according to the formula has problems in actual application, what should we replace it based on?
Answer: The calculation formulas of output chokes and output filter capacitors of different topologies are different. You should choose the appropriate calculation formula according to the circuit structure you choose. The size of the output capacitor is mainly determined by how many millivolts the output ripple voltage should be suppressed to. This requires calculating the ESR, and then you can choose it according to the DATASHEET provided by the manufacturer. However, when selecting a capacitor, you must also consider the change of load, current change range, output inductance, etc., because they will change the characteristics of the capacitor.
87. At present, HID hernia lamps have been widely used in some high-end car headlights, but in the high-voltage circuit design of HID lamp ballasts, it is found that the high-voltage recovery speed is not fast enough, which sometimes causes poor lighting. How to solve it?
Answer: HID lamps generally have a secondary breakdown process, and then the headlights tend to a stable working state; first of all, the secondary breakdown must be effectively controlled to ensure its stable operation. To measure the secondary breakdown, you only need to use the long record length of TDS5000, perform a single trigger to capture its waveform, and then measure the peak voltage and pulse width of the primary and secondary breakdown respectively, and then measure the time between the two breakdown pulses. According to the actual situation, see if the above parameters meet the design requirements.
88. If you use a probe and a virtual instrument, you can display the waveform on the PC. At the same time, various calculations can be easily achieved. What is the essential difference between the TEK5000 series and virtual instruments?
Answer: Although DS5000 is an oscilloscope based on Windows 2000, it is actually divided into two important parts. First, it has a real oscilloscope acquisition and processing part. The data processing of this part is carried out by a professional processor of the oscilloscope itself, while the computer platform of Windows 2000 only performs some background analysis and calculation processing on the data collected by the oscilloscope (internal communication through the PCI bus), which has nothing to do with the display of the oscilloscope itself.
The so-called virtual instrument (mostly PC plug-in card type) collects external signals into the computer through a data acquisition card (generally very slow) and processes the data through the computer's own CPU. It is a cheap solution, but its fatal weakness is that it has no traceability (it is too affected by the computer host, and the test results caused by different hosts have large errors). We know that the consistency of the test instrument is the key to determine the success or failure of the test results.
89. How to reduce the heat loss of DC-DC transformers? What issues should be paid attention to when designing transformers? What are the requirements for the peripheral circuits of transformers?
Answer: The principle of flux reset should be followed. To design a transformer, you need to select the core specifications and size, calculate the duty cycle, magnetic induction increment, and the number of turns of the primary and secondary sides. In the experiment, check the worst-case magnetic saturation.
90. The most difficult problem often encountered in the design of switching power supplies is the efficiency problem. The efficiency of the whole machine depends largely on the loss of the switch tube. After our circuit and device are selected, it is very important to measure the switching waveform of the switch tube. We can judge and improve the working state of the switch based on its data. So how should we operate correctly and pay attention to what problems when using an oscilloscope to perform this test?
Answer: There are two major themes in switching power supplies: improving efficiency and improving reliability. Efficiency requires measuring losses, which are mainly concentrated on the switch tube and magnetic components. For this reason, we should use an oscilloscope to measure the turn-on loss, cut-off loss, and conduction loss. Similarly, the core loss and dynamic inductance of transformers and inductors can be measured.
91. In actual work, it is necessary to test and analyze the switch oscillation signal, video signal, etc. How to do it?
Answer: TEK's TDS5000 series oscilloscope can easily measure and analyze these two types of signals.
For the driving signal of the switching power supply you mentioned, our TDSPWR2 provides four types of analysis: duty cycle trend analysis, switching frequency trend analysis.
Width and cycle trend analysis: TDS5000 oscilloscope has more abundant video triggers, can apply multiple formats, can trigger the field separately and in parallel.
92. When using a transformer algorithm in a flyback switching power supply, it always needs to be adjusted many times. Is there a more universal transformer parameter calculation method for flyback switching power supply?
Answer: Although the design of the transformer is calculated by theory, it still requires multiple tests and adjustments due to the differences in the magnetic core, winding method, etc. Generally, the primary inductance is calculated first, the magnetic core material and skeleton size are selected according to the output power, and then some parameters such as the cross-sectional area of the magnetic core are determined according to the manual. The purpose of single-ended transformer design is to reset the magnetic flux of the magnetic core.
93. Using TDS3032B and THS710 oscilloscopes, how to completely capture and store a one-time random signal, and then replay and analyze it?
A: If the so-called random signal is a single signal, then you only need to set the vertical and horizontal scales that match the signal, adjust the trigger level, use the single trigger to wait for the signal to appear, and then use SAVE/RECALL to store it in ref and call it out at any time; if the signal is an abnormality in a repetitive signal, you can first Autoset, then set
the acquisition mode to fast 500-point display, and adjust the afterglow to infinite.
94. What are the special requirements for the switching power supply to start at low temperature (such as: below -20℃)?
A: The key is the temperature range of the device selection. For example, capacitors, MOSFETs, diodes, etc.
95. The switching power supply will always have electromagnetic radiation, and it is more likely to be interfered by other electrical equipment. How can we achieve the goal of not being interfered by other electrical appliances and effectively radiating the device outward?
A: Because the switching power supply works in a high-voltage and high-current switching state, the electromagnetic compatibility problems caused by it are quite complicated. From the perspective of the electromagnetic compatibility of the whole machine, there are mainly common impedance coupling, line coupling, electric field coupling, magnetic field coupling and electromagnetic wave coupling. The three elements of electromagnetic compatibility are: interference source, propagation path and interfered object. Common impedance coupling mainly refers to the common impedance between the interference source and the interfered object, through which the interference signal enters the interfered object. Inter-line coupling mainly refers to the mutual coupling caused by parallel wiring of the conductors or PCB lines that generate interference voltage and interference current. Electric field coupling mainly refers to the coupling of the induced electric field on the interfered object due to the existence of potential difference.
Magnetic field coupling is mainly the coupling of low-frequency magnetic field generated near the pulse power line of large current to the interference object. Electromagnetic wave coupling is mainly due to the high-frequency electromagnetic wave generated by the pulsating voltage or current, which radiates outward through space and produces coupling to the corresponding interfered body. In fact, each coupling mode cannot be strictly distinguished, but the emphasis is different. From the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility of switching power supply, we can start from three aspects.
1) Reduce the interference signal generated by the interference source;
2) Cut off the propagation path of the interference signal;
3) Enhance the anti-interference ability of the interfered body.
When solving the electromagnetic compatibility inside the switching power supply, the above three methods can be used in combination,
with the cost-effectiveness ratio and the difficulty of implementation as the premise. The external interference generated by the switching power supply, such as power line harmonic current, power line conduction interference, electromagnetic field radiation interference, etc., can only be solved by reducing the interference source.
On the one hand, the design of input and output filter circuits can be enhanced, the performance of active power factor correction (APFC) circuits can be improved, the voltage and current change rate of the switch tube and the rectifier freewheeling diode can be reduced, and various soft switching circuit topologies and control methods can be adopted.
On the other hand, the shielding effect of the casing should be strengthened, the gap leakage of the casing should be improved, and good grounding should be carried out. For external anti-interference capabilities, such as surges and lightning strikes, the lightning protection capabilities of the AC input and DC output ports should be optimized. Usually, for the combined lightning waveform of 1.2/50μs open-circuit voltage and 8/20μs short-circuit current, due to its small energy, a combination of zinc oxide varistor and gas discharge tube can be used to solve it.
To reduce the internal interference of the switching power supply, realize its own electromagnetic compatibility, and improve the stability and reliability of the switching power supply, the following aspects should be taken into consideration:
pay attention to the correct distinction between the PCB wiring of digital circuits and analog circuits, and the correct decoupling of the power supply of digital circuits and analog circuits;
pay attention to the single-point grounding of digital circuits and analog circuits, and the single-point grounding of large current circuits and small current circuits, especially current and voltage sampling circuits, to reduce common impedance interference and reduce the influence of ground loops;
pay attention to the spacing between adjacent lines and signal properties when wiring to avoid crosstalk; reduce ground line impedance; reduce the area surrounded by high-voltage and high-current lines, especially the primary side of the transformer and the switch tube, and the power supply filter capacitor circuit;
reduce the area surrounded by the output rectifier circuit and the freewheeling diode circuit and the DC filter circuit; reduce the leakage inductance of the transformer and the distributed capacitance of the filter inductor; use filter capacitors with high resonant frequency, etc.
The power test solution launched by TEK can perform pre-consistency tests on current harmonics according to the EN61000-3-2 standard.
96. What data is the SOA test obtained from, and what measurement method of the oscilloscope can be used to obtain this data?
Answer: SOA is the safe operating area measurement, which is used to determine the reliability of power devices. When a short circuit or power-on occurs, the safe operating area may be exceeded for only a few cycles, and this is not easy to detect. The impact on the device will not damage it, but it is also an accumulation for the device, and the device margin may not be enough.
97. How to test the jitter component with an oscilloscope?
Answer: Deterministic jitter can be measured with an oscilloscope. The time width of the rising/falling edge can be read on the oscilloscope. According to the signal period, it can be converted into UIp-p, which is the peak amplitude of the jitter, as shown in the figure below. For more details, please refer to the relevant information of oscilloscope manufacturers such as Tektronix.
98. How to distinguish between analog bandwidth and digital real-time bandwidth?
Answer: Bandwidth is one of the most important indicators of an oscilloscope. The bandwidth of an analog oscilloscope is a fixed value, while the bandwidth of a digital oscilloscope has two types: analog bandwidth and digital real-time bandwidth. The highest bandwidth that a digital oscilloscope can achieve for repetitive signals using sequential sampling or random sampling technology is the digital real-time bandwidth of the oscilloscope. The digital real-time bandwidth is related to the highest digitization frequency and the waveform reconstruction technology factor K (digital real-time bandwidth = highest digitization rate/K), and is generally not given directly as an indicator. From the definitions of the two bandwidths, it can be seen that the analog bandwidth is only suitable for the measurement of repetitive periodic signals, while the digital real-time bandwidth is suitable for the measurement of both repetitive signals and single-shot signals. When manufacturers claim that the bandwidth of an oscilloscope can reach so many megabytes, it actually refers to the analog bandwidth, and the digital real-time bandwidth is lower than this value. For example, the bandwidth of TEK's TES520B is 500MHz, which actually means that its analog bandwidth is 500MHz, while the highest digital real-time bandwidth can only reach 400MHz, which is much lower than the analog bandwidth. Therefore, when measuring a single-shot signal, it is necessary to refer to the digital real-time bandwidth of the digital oscilloscope, otherwise it will bring unexpected errors to the measurement.
99. Can an oscilloscope be used as a digitizer?
Answer: The fastest oscilloscopes and digitizers usually use parallel flash converters and 8-bit resolution. 8 bits or 256 levels of digitization are sufficient to express a relatively smooth and easy to understand waveform display. Therefore, why not use a digital storage oscilloscope (DSO) as a digitizer, especially for high-speed signals, it is difficult for both instruments to obtain resolutions above 8 bits.
In fact, the results of doing so are satisfactory, but there are exceptions. Oscilloscopes are non-continuous acquisition instruments and digitizers are not like that. After the oscilloscope captures a signal, it needs a place to put the data before capturing more signals, unless continuous waveform acquisition similar to the frame rate of a TV is used to store the data in a pixel image. Such acquisition and equivalent display rate is very high, but the data format makes the amount of data for further external analysis very large.
In addition to the special processing mentioned above, oscilloscopes can only continuously acquire and display signals at very low speeds. Digitizers can achieve continuous throughput rates of 100MS/s or higher, limited only by the speed of the memory bus. For example, a digitizer card for the PCI bus has a data transfer rate of 100MB/s, and the PCI bus can work up to 66MS/s (132MB/s).
The throughput rate of an oscilloscope is limited by the data processing speed of the slower, low I/O capabilities. Slower digitizers and data recorders can write data directly to the hard disk and archive several GB of data, while oscilloscopes generally only have a maximum of 16MB. If you look at the data transfer rate from another perspective, many applications only need to capture sporadic data, but these bursts may be very close. At this time, it is very important to transfer data records quickly. Such signals include high repetition frequency (PRF) scanning radar, time-resolved ultrasonic sonar, time-of-flight mass spectrometers, and nuclear counting applications.
100. What is a combination oscilloscope?
Answer: A combination oscilloscope is an oscilloscope that combines the capabilities and advantages of both an analog oscilloscope and a digital storage oscilloscope (DSO). When the combination oscilloscope is set as a DSO, the user can use it to perform automatic parameters, measurements, store acquired waveforms and then make hard copies; at the same time, when needed, it can also have the infinite resolution of an analog oscilloscope and the familiar and reliable waveform display. When using the combination oscilloscope, no matter how high or low the signal repetition rate is, the brightest display can be obtained.
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