100 super practical questions and answers on the basics of oscilloscopes
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1. How to use an oscilloscope to detect and analyze the reliability of a designed product?
A: Oscilloscopes have long been one of the most effective tools for testing electronic circuits. By observing the voltage and current waveforms at key nodes of the circuit, one can visually check whether the circuit is working properly and verify whether the design is appropriate. This is extremely helpful in 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 an oscilloscope probe?
A: 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 RMB to nearly $10,000. 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 passive probes must have a lot of passive device compensation circuits (RC networks) inside.
3. How long is the service life of a Keysight oscilloscope probe? Do probes need to be calibrated regularly?
A: It is hard to say how long the probe of an oscilloscope will last, as 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 switching 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 require zero drift adjustment.
4. What is the real-time sampling rate of an oscilloscope?
A: 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 equivalent time sampling of an oscilloscope?
A: Equivalent time sampling means that the oscilloscope combines 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 inverse of the minimum sampling interval between the two points is called the equivalent sampling rate. Its index can be very high, 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 represented by COSΦ. In fact, the simplest way to measure it is to measure the phase difference between voltage and current, and the result is the 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 condition of a high-frequency transformer or inductor core?
Answer: The power test solution launched by TEK has a function called BH curve analysis, which can reflect the working status of the magnetic core, measure the dynamic inductance value, and derive 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 them with an oscilloscope?
A: TEK's TDS5000 oscilloscope has frequency domain analysis. By analyzing the frequency band of the noise, the type of noise can be analyzed and the corresponding processing method can be used. The oscilloscope can only provide data analysis and band shape display.
10. How can I use a LeCroy oscilloscope to test the radiation of the power supply?
A: Switching power supplies have radiated interference. The general approach is to find out the source of interference and then shield it. An oscilloscope can be used to analyze its frequency component composition through Fourier transform function, and the type of interference can be determined 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. The method of winding the secondary with the primary in the middle is still not ideal. Are there any techniques for winding the transformer?
Answer: Wind the high-power output winding inside, as close to the primary side as possible to enhance coupling.
12. Is there an oscilloscope that can analyze switching losses?
A: Tektronix's power supply test system, namely the TDS5000 series digital phosphor oscilloscope plus TDSPWR2 power analysis software, can easily analyze switching losses and power losses per cycle, even including RDS ON.
13. Can an oscilloscope perform Fourier decomposition?
Answer: Most modern digital oscilloscopes have FFT functions, and the above-mentioned system can even perform pre-tests on current harmonics according to the EN61000-3-2 standard.
14. Can an 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 signals such as PWM that follow the trend of pulse width changes and are similar to sine waves.
15. When using a digital oscilloscope, what are the principles for setting the B trigger and trigger level and the measured signal?
A: Tektronix oscilloscopes support A, B trigger function, which means dual event sequence triggering. When AB seq is selected, A event is used as the main trigger to capture complex waveforms in conjunction with B event. The trigger method is A event arm trigger system, and when the defined B event occurs, it triggers at the B event. For detailed triggering instructions, please refer to the oscilloscope manual.
16. How to use TDS3052B to measure the maximum value of a modulated wave with a carrier frequency of tens of K and a modulating wave frequency equal to the power supply frequency?
A: The power frequency input may be a low frequency of 50Hz/60Hz, and the carrier is tens of K. One 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 -3dB point where the sine wave amplitude attenuates.
② The depiction of waveforms and rise times in digital oscilloscopes is achieved by obtaining waveform data through real-time sampling circuits and high-speed A/D converters, and then through interpolation operations.
③ In Tektronix oscilloscopes, 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 oscilloscope's main processor, which will take more time.
④ For the signal you are measuring, I am afraid that it is impossible to use a 100MHz oscilloscope. For a 50MHz square wave, theoretically, an oscilloscope of 450MHz or higher should be used to accurately reproduce the most important harmonics below the 9th order in the signal, thereby ensuring that the waveform is not distorted. Moreover, 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 goes for probes. Since ordinary probes will produce high-frequency distortion when measuring high voltage, you should use special differential probes or high-voltage probes such as Tektronix's P5205 and P5100 for measurement.
18. How to use a digital oscilloscope effectively in analog circuits, such as measuring small signals of audio amplifiers and noise in power supplies?
A: Issues to note are:
① The grounding problem of the oscilloscope. The reference ground wire of the oscilloscope case and the probe are both connected to the ground wire, so good grounding is the primary condition for measuring interference.
② The interference problem introduced by the reference ground wire of the oscilloscope. Since ordinary probes usually have a ground wire, it 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 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, which provides a bandwidth of up to 5GHz.
④ Many Tektronix oscilloscopes provide a high-resolution (Hi-Res) signal capture mode that can filter out random noise superimposed on the signal.
19. When measuring the conducted disturbance of the off-board signal line, 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?
A: There are several factors to consider when an oscilloscope can test noise signals:
① The amplitude of the measured signal, whether it is a small signal, the oscilloscope with the probe can test uA level signals.
② The frequency of the signal being measured.
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?
A: 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 mainly set for large-cycle repetitions and there are many non-repetitive waveform points that meet the trigger conditions within the large cycle. For example, as shown in the figure, the red points in the figure can 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 it can be displayed stably.
In addition, trigger holdoff is also used for amplitude modulated signals, etc. For more details, please refer to the Tektronix article "Oscilloscope XYZ".
21. Regarding holdoff, what is the difference between triggering and non-triggering in the oscilloscope's processing of acquired signals?
A: For a digital oscilloscope, whether it is triggered or not, the oscilloscope is actually constantly collecting waveforms, but only a stable trigger can produce a stable display. 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 condition is met. If the "Normal" mode is used, the waveform will not be displayed if the trigger condition is not met.
22. Regarding holdoff, if the horizontal time resolution remains unchanged, does a larger percentage setting (corresponding to the signal display being gradually stable) mean a longer signal period?
A: Yes, the larger the percentage, the longer the hold-off 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 at this time. If there is no differential probe, you can use two differential probes to connect to the two channels of the oscilloscope (such as Ch1 and Ch2). Then use mathematical operations to obtain the ch1-ch2 waveform 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?
Answer: There are two types of USB signal testing:
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, mainly to 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 select a suitable differential probe to connect to D+ and D- to 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 fidelity of the signal test, you need to select an oscilloscope greater than 2GHz and a differential probe for testing.
25. Characteristics of high-speed signals on PCB board: XAUI interface 3.125GBd serial differential signal: 60ps, what bandwidth of oscilloscope is needed to accurately measure? What is the possible measurement error?
Answer: The 3.125GBd serial differential signal of the XAUI interface sounds a bit like an InfiniBand signal. It is collected using sinusoidal interpolation or similar equivalent sampling methods. However, due to factors such as its own 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 measurements of < 80ps, the error will be greater than 10%. Although this is already the best solution for real-time oscilloscopes, the most accurate solution for rise time measurement is Keysight's network analyzer (which needs to be equipped with physical layer analysis software) because its bandwidth can be as high as 50GHz.
26. For designs that have very high requirements for the phase noise parameters of the clock, what key issues need to be considered to reduce the phase noise?
Answer: There are many indicators to measure the performance of ADC and DAC devices: number of bits, conversion speed, DC accuracy, switching performance, dynamic performance (SNR, SINAD, IMD), etc.
27. How to measure the phase noise in a design that has very high requirements on the phase noise parameters of the clock?
Answer: From the perspective of an oscilloscope, you can test the amplitude, time, converted signal quality, conversion speed, clock and data setup/hold time and other parameters of the analog and digital signals of ADC and DAC. You can also use the advanced calculation function (spectrum analysis function) in the TDS oscilloscope to qualitatively measure parameters such as SNR and SINAD.
28. Since it may be necessary to introduce an external clock, there is a 2-choose-1 problem in the clock. What solution can be used to minimize the deterioration of phase noise?
Answer: First, we need to analyze the source of jitter. An oscilloscope is a good tool for analyzing jitter. Currently, we can use the TDS5000B/6000B/7000B series oscilloscope with jitter analysis software to perform a thorough jitter analysis, such as determining jitter (Dj), random jitter (Rj), and separating Rj and Dj. Finally, we can eliminate jitter by analyzing the causes of jitter.
29. When viewing waveforms on an oscilloscope, what is the difference between using external triggering and self-triggering?
A: The oscilloscope is usually triggered by 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 self-triggering of the signal, and there is no better question. 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 required. For 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 a trigger signal, and the oscilloscope will be able to display the entire cycle.
30. The bandwidth of TDS3032B is 300MHz, and the sampling frequency is 2.5G/s, which is 8 times the bandwidth. What is the fixed relationship between bandwidth and sampling frequency? We also have an oscilloscope from another manufacturer, such as Tektronix oscilloscope. The bandwidth is 100MHz and the sampling frequency is only 200MHz. Why is the bandwidth sampling frequency ratio of the two oscilloscopes so different?
A: 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 satisfy the Nyquist sampling theorem, that is, theoretically at least 2 points should be sampled for each cycle of the highest frequency signal of the measured signal, otherwise aliasing will occur. 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-shot signal) = repetition 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?
A: Bandwidth is a basic indicator of an oscilloscope. It is the same as the definition of amplifier bandwidth, which is the so-called -3dB point, that is, the frequency point when the amplitude of the oscilloscope input plus a sine wave is attenuated to 70.7% of the actual amplitude 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 (oscilloscope below 1GHz).
33. Under the condition of a certain bandwidth, is it meaningless to have a too high sampling frequency?
A: Bandwidth is the basic condition for limiting the capture of high-frequency components of the measured signal. Using a Tektronix oscilloscope, only 2.5 points are needed per cycle of the measured signal 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.
34. What are the advantages and disadvantages of the so-called Gaussian response oscilloscope and flat response oscilloscope and their respective applicable fields?
A: There are no indicators of flat response and Gaussian response in the specifications of oscilloscopes. There may be similar comparisons or discussions 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 TDS7000 4GHZ bandwidth oscilloscope launched by Tektronix in 2000, the TDS6000 6GHZ bandwidth oscilloscope launched in 2001, the TDS7704B 7GHZ bandwidth oscilloscope launched 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 remedy the lack of bandwidth of analog devices and obtain higher bandwidth, and the frequency response curve will naturally change.
As more and more high-speed signals are being generated and the signal rates are getting faster, new requirements are being put forward for real-time oscilloscopes. Oscilloscope manufacturers have also seen some new technologies in their digital oscilloscopes. The most notable one is that oscilloscopes use digital signal processing technology (DSP) to achieve better performance.
The main applications of filters include:
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Gain and waveform calibration and improvement
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Amplitude and phase improvements
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Optical Reference Receiver Normalization
Among them, 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 "Can bandwidth 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 Tektronix's latest 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?
A: The frequency response of the oscilloscope preamplifier is the most critical factor in determining the test results. It is determined by the analog device. 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 real 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?
A: Your P6139A probe plus Tektronix's 500MHz oscilloscope can still achieve a typical bandwidth of 500MHz, but as you said, the input capacitance is different, and this capacitance will produce a load effect on the signal to be measured, causing signal ringing and shape changes. Therefore, using an active probe can reflect the true situation of the signal. In fact, when using a probe, we must consider not only the bandwidth, but all these factors when measuring high-frequency signals:
Especially for P6139A, you also need to 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 are using is an active differential probe, and the influence of common mode may also be a factor. Passive probes are mainly chosen because of their large dynamic range. For example, the P6139A can measure signals from millivolts to hundreds of volts, while the P6247 can only measure +-8.5V signals. In addition, the price of active probes is also a factor.
37. During the experiment, the oscilloscope was connected to the ground wire, which caused the MOSFET to blow up. Now the ground wire of the oscilloscope has been cut off. What is the reason?
A: To ensure personal safety during testing and obtain good measurement results, the ground wires of all probes of the oscilloscope are generally connected to the chassis and to the ground wire of the oscilloscope power cord. Therefore, when you measure the MOSFET tube waveform in the power supply, if any point is not the ground, there will be problems, as shown in the figure below.
Cutting the ground wire can prevent short circuit problems in MOSFET tube testing, but it will also bring some other testing problems, such as the oscilloscope case being charged, and the oscilloscope case distributed parameters affecting the measured signal. The solution is to use a differential probe, such as Tektronix's P5205, which can measure the so-called differential signal when neither of the two test points is ground.
38. When capturing data with an oscilloscope, I found that the stored text only contains the data on the current screen, and the time interval is based on the resolution. How can I use software to process the data in real time (matlab?), and how can I capture more data?
A: 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, and 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, forming 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 memory depth, such as the TDS5000B series general-purpose oscilloscope, which can support up to 16M memory.
39. What are the factors that affect the operating speed of Yokogawa oscilloscope?
A: In fact, the principle of any oscilloscope is similar. The front end is a data acquisition system, and the back end is computer processing. There are two main factors that affect the speed. One is the data transmission from the front end data acquisition to the back end processing. Generally, the PCI bus is used. This is a transmission bottleneck, but new technologies have been developed to 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?
A: The processing speed will be slow if the amount of data is large. 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, I can always measure a very strong 50Hz AC but no signal. However, the ground of the oscilloscope is consistent with the ground of the parallel port being measured. What should I do?
A: You can start from the following aspects:
① Check whether the oscilloscope is well grounded or isolated by an isolation transformer;
② Is there a strong 50Hz signal sensor nearby?
③ In a strong interference environment, you should pay attention to whether the parallel port's driving capability and operating frequency are appropriate for the test operation selection. If you only see a 50Hz interference sine wave, and the waveform is relatively regular, you should consider that the parallel port may not be working;
④ Check whether 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 need to 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, I 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 and the ground are connected together, there is an interference signal, no matter where I measure. The interference signal is audio. Why is this?
A: Issues to note are:
① The grounding problem of the oscilloscope. The reference ground wires of the oscilloscope housing and the probe are both connected to the ground wire, so good grounding is the primary condition for measuring interference;
② The interference problem introduced by the reference ground wire of the oscilloscope. Since ordinary probes usually have a ground wire, it 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 use the method of removing the probe cap and not using the ground wire led out from the probe. Instead, use the probe tip and the ground inside the probe to directly contact the point to be measured for measurement;
③ Use differential measurement 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, which provides a bandwidth of up to 5GHz;
④ Many Tektronix oscilloscopes provide a high-resolution (Hi-Res) signal capture mode that can filter out random noise superimposed on the signal.
43. In EMC tests, the indicator meter sometimes 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 using the oscilloscope's high-frequency suppression trigger mode, limiting the oscilloscope's bandwidth, etc.
44. Why can’t an oscilloscope capture the amplified current signal sometimes?
A: If the signal does exist, but the oscilloscope can sometimes capture it and sometimes not, this may be related to the oscilloscope settings. 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 Yokogawa oscilloscope be used for microcontroller development?
A: In the process of developing single-chip circuits, 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, which can directly debug the communication on the serial bus. In addition, if you use DSP combined with MCU to develop circuit boards, 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 storage depth of 2M points per channel is very helpful in analyzing the cause of the problem, observing long-term serial signals, observing handshake timing, etc. And its amplification function can amplify the signal tens of thousands of times to observe details.
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 detection ?
Answer: The general working speed of I2C Bus signal does not exceed 400Kbit/s. Recently, chips with several Mbit/s have appeared. When setting the trigger conditions for 54621A and 54621D, there is no need to consider the impact of different speeds. However, for other buses, such as CAN bus, you must first set the actual working speed of the CAN bus on the oscilloscope so that the oscilloscope can correctly decode the protocol and trigger correctly.
47. In addition to 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 Keysight's logic analyzer, but its price is relatively higher than that of 54621A/D.
48. What kind of signals are suitable for testing various triggering applications of digital oscilloscopes, such as edge triggering, glitch triggering, and pulse width triggering?
Answer: ① Edge trigger, edge trigger, can set the trigger level, rising edge or falling edge. Edge trigger is also called basic trigger.
② Advanced trigger, which covers a variety of trigger functions. You can set the corresponding trigger conditions according to the characteristics of the measured signal to 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 smallest glitch that an oscilloscope can capture is the sampling rate of the oscilloscope. Do all oscilloscopes follow this rule? In this case, will the pre-filter of the oscilloscope have an impact on it?
A: It cannot be asserted that all oscilloscopes are like this. For example, some oscilloscopes reach 1GS/s, but 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 the 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?
A: If the burr is 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 mode, which usually synchronizes the signal. If the signal itself has burrs, but you want the oscilloscope to filter out the burrs and not display the burrs, it is usually difficult to do so.
You can try to limit the bandwidth, but if you are not careful, you may filter out some information from the signal itself. If you use a logic analyzer, generally speaking, using the state acquisition method, some glitches acquired in the timing mode will not be visible.
51. In actual work, when encountering sudden glitch signals, how to capture and test them?
Answer: For example, when we are performing clock testing, we often encounter occasional glitch signals, which will cause our circuit to malfunction. 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 oscilloscope's advanced trigger function - pulse width trigger according to 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 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, edge triggering cannot be used to correctly synchronize signals, and 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 this abnormality is sporadic, 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, and at the same time, it can generally be seen but not measured). If the pulse width of the object to be measured is 50ns, and there is no problem with the signal, that is, there is no signal distortion or narrower caused by interference, competition, etc., the signal can be synchronized using edge triggering without using glitch triggering. Many users set the pulse width trigger to 10ns ~ 30ns. Fortunately, 5462x and 546?x 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 triggering, and you may use the 5ns setting.
53. Does Keysight Technologies' digital oscilloscope have a DPO function?
A: DPO is a special term. Only one oscilloscope company uses this term. The corresponding function of Keysight Technologies is called MegaVision. The similarities between it and 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:
①After discovering an abnormal signal, MegaVision can directly magnify the anomaly 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 546?xA/D oscilloscopes) or even higher. ③MegaVision oscilloscopes are optimized for applications that require deep storage. When the oscilloscope storage depth is >10K, 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 this bandwidth only to achieve no sampling aliasing error?
Answer: Determine the sampling rate after determining the bandwidth. Some formulas in the industry do determine the sampling rate in order to achieve no sampling aliasing error, but it is a general assessment statement. It depends on the characteristics of your object under test, 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 of 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 a point at the trough on both sides, a triangle wave will be displayed, while the sine interpolation display will still be a sine wave. Therefore, some application articles say: 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. Characteristics of high-speed signals on PCB: 156.25MHZ differential clock signal, Rise/Fall Time (20%～80%) <100ps, jitter tolerance (pp<30ps, RMS<2ps), skew (+ vs.-) <20ps. How high bandwidth oscilloscope is needed for accurate measurement? What is the maximum measurement error?
A: 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 tested. Fortunately, Keysight has launched a 7GHz bandwidth differential probe. At the same time, the actual rise time indicator of 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/Ds or 5GSa/sA/Ds) to piece together a 20GSa/s.
57. When choosing an oscilloscope, bandwidth is usually the most important consideration. So, under what circumstances should the sampling rate be considered?
A: It depends on the object being measured. Under the premise of satisfying 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. 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 method: 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)?
A: This phenomenon is related to the oscilloscope display. The traces seen on an analog oscilloscope are generally thinner. It directly applies voltage to the screen through a vertical deflector, and the scanning rate and waveform refresh rate are very fast. A digital oscilloscope quantifies the waveform voltage through A/D, stores it in memory, and displays it after processing. The display resolution of a digital oscilloscope screen is limited, usually 6?0 points or 1000 points. If you set the oscilloscope's storage depth (record length) to 10K or 2M, this means that the amount of information of 10K or 2M points in the memory must be reflected through 6?0 points or 1000 points. No matter how good the algorithm is, it will bring about certain display errors. The degree of waveform thickening is related to the storage depth. These problems are unique to digital oscilloscopes. In addition, the default display mode of a digital oscilloscope is vector display mode, which means that some points will be inserted between two sampling points using a linear algorithm or a sine interpolation algorithm. 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 each sampling of the digital oscilloscope, adjust the time base, and you can clearly see these points, even if the vector display is used, the line will become thinner. From the perspective of the instrument, in addition to measuring small signals, the results of using a 1:1 probe may be better than 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. When it comes to observing waveform details, analog or digital oscilloscopes, which one is more advantageous (for example, observing parasitic waveforms below 1% at zero crossings and peaks)?
Answer: When observing parasitic waveforms below 1%, the accuracy of both analog and digital oscilloscopes is not very good. The vertical accuracy of analog oscilloscopes is not necessarily higher than that of digital oscilloscopes. For example, the vertical accuracy of an analog oscilloscope with a bandwidth of 500MHz is +/-3%, which is not more advantageous than a digital oscilloscope (usually with an accuracy of 1~2%). In addition, the automatic measurement function of a digital oscilloscope is more accurate than the manual measurement of an analog oscilloscope in terms of details.
60. Digital oscilloscopes generally provide online display of RMS values. What is their accuracy?
Answer: Many people measure the amplitude measurement accuracy of an oscilloscope by the number of A/D bits. In fact, it will vary 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 large. If the bandwidth is sufficient and the sampling rate is set very high, the actual amplitude measurement accuracy will not be as good as when the sampling rate is low (you can sometimes refer to the user manual of the oscilloscope, which may give the actual number of effective bits of the oscilloscope's A/D at different sampling rates). In general, the accuracy of an oscilloscope in measuring amplitude, including the RMS value, is often not as good as that of a multimeter. Similarly, when measuring frequency, it is not as good as a frequency counter.
61. How to capture and reproduce the fleeting instantaneous signal?
A: 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 Keysight 5462xA/D, 546?xA/D, 5483xB/D, 5485xA, these instruments all support the MegaZoom function, that is, you can zoom in on local details while observing the overall signal, 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 used to acquire the signal and the maximum sampling rate that can be used.
62. Which Keysight oscilloscope can test a carrier signal with a frequency of 500M?
A: 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 (Keysight 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 you must use a probe, it is recommended to use the 1157A active probe (2.5GHz bandwidth). If you use an oscilloscope with a 500MHz bandwidth, even if you use a BNC cable, the best case amplitude measurement error is 29.3%, and the rise time measurement accuracy is 58.6%.
63. The oscilloscope is nominally marked as 60 MHZ. Can it be understood that it can measure a maximum of 60 MHZ?
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 I measure a 4.1943MHZ square wave using an oscilloscope rated at 60MHZ?
A: If the signal to be tested is a square wave, the reference standard for selecting an oscilloscope is the rise time of the signal. If the bandwidth of the oscilloscope = 0.35/signal rise time * 3, the rise time measurement error is about 5.4%.
The bandwidth of the oscilloscope probe 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 probe with a bandwidth of 20MHz, 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 use a BNC cable to connect a function generator to check whether there is any problem with the oscilloscope itself or the probe. If there is any problem, you can contact the manufacturer directly.
65. How to measure the stability of a clock?
A: If you are using 5483xB/D, 548xxA, 5484xB or 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 the application article of Keysight Technologies: "Jitter Analysis Techniques Using an Keysight Infiniium Oscilloscope" (P/N: 5988-6109EN), which can measure the jitter in the worst case. For 5485xA, if you want more powerful jitter analysis function, it is equipped with special jitter analysis software, which provides very powerful jitter analysis. For details, please refer to the Datasheet of 5485x oscilloscope. For more detailed information, please call Agilent.
66. What are the methods and techniques for accurately measuring period jitter in PLL using a Keysight oscilloscope?
A: If you are using 5483xB/D, 548xxA, 5484xB and 5485xA, you can use the standard configuration histogram method to measure the jitter of the clock edge or amplitude. For details, please refer to the application article "Jitter Analysis Techniques Using an Keysight Infiniium Oscilloscope" (P/N: 5988-6109EN) of Keysight Technologies, 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. When using the oscilloscope, you should pay attention to whether its own jitter-related indicators meet the test requirements, such as the trigger jitter indicators of the oscilloscope itself, etc. At the same time, you should 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 Keysight oscilloscope to measure the Settle time of PLL?
A: This can be done using a Keysight 548xx series oscilloscope + USB-GPIB 82357A adapter + software option. This can also be done using Keysight's lower priced modulation domain analyzer.
68. When designing a PLL, how do you measure the dead zone of the PFD (Phase Frequency Detector)?
A: 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 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. The setup and hold time setting at this time corresponds to your PFD dead zone. In theory, it is believed that loss of lock will occur at two moments. One is at the initial working time, when the difference (frequency difference) between 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 difference between the two signals to exceed the tracking bandwidth of the PLL and lose lock. All Agilent 548xx series oscilloscopes can complete this measurement (assuming the bandwidth is sufficient).
69. How to test optical signals using Keysight equipment?
A: Keysight Technologies has a complete 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?
A: 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 use the oscilloscope's FFT function to analyze from the frequency domain. 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 zoom in on the waveform to display it flat. When measuring power supply ripple, you can use the Zoom function to zoom in on the ripple part for analysis; in addition, you may consider analyzing the power supply from a frequency domain perspective to observe its harmonics and clutter. 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 standardly equipped with software that connects to a computer, directly fetching data into the computer for further analysis. Of course, you can also install Matlab software directly into the 548xx. If you already know the parameters of the circuit, you can directly adjust the oscilloscope settings to make it work at the appropriate sampling rate and vertical scale.
71. The ripple of the switching power supply output voltage is an important indicator. How to use an oscilloscope correctly to measure this indicator?
A: Ripple is defined as a clutter signal containing periodic and random components attached to the DC level, 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, so 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. Here, 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. For example, whether the measurement frequency should be limited to below 20MHz.
72. When measuring ripple, a large part of it is a 50 Hz periodic sharp pulse. The greater the load current, the greater the pulse amplitude. What are the specific solutions?
A: In the Tektronix power measurement system, when performing ripple measurement, we can choose power frequency ripple test or 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 out other frequency components.
73. How to remove the noise on the ripple, such as power frequency noise, when measuring ripple?
A: The noise on the ripple can be removed by using the high-resolution capture mode of the TDS5000 oscilloscope in the capture mode. 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 types of ripples.
Measure the results separately after separation.
74. To accurately test the ripple and noise of a switching power supply, is it necessary to do so in a dedicated laboratory?
A: Of course, it would be ideal if there is a dedicated laboratory to perform ripple measurement. When this condition is not available, the following issues should be noted:
① The oscilloscope should have a good grounding;
② If the measurement standard requires bandwidth limitation, the 20MHz bandwidth limit in TDS430A should be turned on;
③ Use AC coupling of oscilloscope;
④ Use a BNC cable and measure with the 50 ohm input impedance range of TDS430A (a 50 ohm high-power load, a BNC adapter or a test fixture may be required at this time). To improve measurement accuracy, the oscilloscope probe should not be used, as 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 verify it with an oscilloscope? Even if the probe of an ordinary oscilloscope is directly connected to the probe ground clip, the noise is 20 to 30 millivolts.
A: This is a very representative question. You need to use a voltage differential probe with a high common-mode rejection ratio, which can work in a high-noise environment.
76. How to view and read the period of the displayed waveform using a digital oscilloscope?
Answer: All digital oscilloscopes support waveform period measurement. From the perspective of improving test accuracy, if you are using 5462x/546?x (except 546?5), you can select Counter in its measurement parameters, and its embedded hardware frequency counter will be activated for accurate frequency measurement (5digit). If you are using other types of oscilloscopes, try to let the oscilloscope screen display one cycle of the signal, and the amplitude should be as full scale as possible. At this time, the measurement accuracy is generally better, and you can use the oscilloscope's automatic measurement function or manual measurement with cursors.
77. During development, I encountered a problem. I added functions to the prototype and tested the prototype’s audio, data output, trigger signal, etc. The test results were almost the same as the design results. Why is the prototype’s sound clear and accurate, while the sound of the finished product is sometimes acceptable but sometimes not?
A: The actual sound of the object under test is sometimes acceptable and sometimes not, but the waveform display on the oscilloscope does not show any problems, or the data displayed on the oscilloscope is far different from the data on the object under test. This is often caused by the oscilloscope and your object under test not being synchronized. You can try the following method: The sound signal is usually a low-speed signal. You can let the oscilloscope work in rolling mode. When there is a problem with the signal, manually stop the waveform acquisition and analyze it.
Observing sound signals in the time domain is often not comprehensive. Agilent's dynamic signal analyzer is often a better choice, but if you don't have this instrument, you can use 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 triggering and sequence triggering similar to logic analyzers).
78. How to perform clock jitter test using TDS3012 oscilloscope?
A: 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, only infinite persistence can be used to perform cumulative measurements of signals for a relatively long time. 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 the signal frequency or even higher. 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 a Tektronix oscilloscope can be used to see the frequency and amplitude of the radiation of a switching power supply, but are the amplitude values here the same as the values of the certification center? If not, how to convert them? Moreover, if different V/DIV is selected 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 for qualitative analysis, not quantitative analysis, so it is only of reference value. If you want to analyze the spectrum amplitude, you can select the Blackman-Harris window, which will have a better effect. When converting V/div, it will definitely affect the FFT amplitude, 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 make the waveform occupy the entire screen as much as possible (but never exceed the screen), that is, to select a smaller V/div position.
81. Which type of oscilloscope should I choose to effectively improve design efficiency?
A: At the current stage of oscilloscope development, data analysis has been elevated to an important position. Using an oscilloscope is not only to observe waveforms during debugging, but more importantly, it can be used to analyze and calculate device parameters during design, helping you optimize your design. Choosing the most suitable oscilloscope depends on 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 non-linear distortion and other video parameters)?
A: 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?
A: Oscilloscope probes have specific indicators. You can refer to the equivalent impedance-frequency graph 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 oscilloscope (the oscilloscope probe is 250MHz)?
A: When measuring state transitions, you only need to use the oscilloscope's automatic trigger mode to set the voltage and current waveforms to a more ideal display mode. If you use a TDS5000, you can also adjust the resolution knob to set the sampling rate to a suitable level (usually 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 limiting parameters (such as test time, number of samples per time)? How to use a Tektronix oscilloscope to test the state change 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 can one use an oscilloscope to observe the MOSFET Vt/It trajectory?
A: First, the oscilloscope must have a delay correction function between channels, so that basic accuracy can be guaranteed when performing related mathematical operations. Use high-voltage differential voltage probes and current probes for measurement. The power test solution launched by TEK can dynamically observe the entire working process of MOSFET.
86. The selection of output capacitors and output inductors should be determined according to the power supply requirements of the load. Should the values of L and C be applied according to the formulas specified in the datasheet? If the values calculated according to the formulas have problems in actual applications, what should we base our replacement on?
A: The calculation formulas for 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 according to the DATASHEET provided by the manufacturer. However, when selecting capacitors, you must also consider the changes in load, current variation range, output inductance, etc., because they will change the characteristics of the capacitor.
87. At present, HID xenon lamps have been widely used in some high-end car headlights, but in the design of the high-voltage circuit of the HID lamp ballast, it was found that the high-voltage recovery speed was not fast enough, resulting in poor lighting sometimes. How to solve it?
Answer: HID headlights generally have a secondary breakdown process, and then the headlights tend to a stable working state; first, 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 a PC. At the same time, various calculations can be easily performed. 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, and the computer platform of Windows2000 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 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, and 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 determining 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 magnetic flux reset should be followed. To design a transformer, one must select the core specifications and dimensions, calculate the duty cycle, magnetic induction increment, and the number of turns of the primary and secondary sides. In the experiment, the worst-case magnetic saturation condition should be verified.
90. The most difficult problem often encountered in the design of switching power supply 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, the switching waveform measurement of the switch tube is very important, and its data can be used to judge and improve the working state of the switch. So how to operate correctly and what problems should be paid attention to when using an oscilloscope to perform this test?
A: There are two major themes in switching power supplies: improving efficiency and improving reliability. Efficiency requires measuring losses, which are mainly concentrated in 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, for transformers and inductors, we can measure their core loss and dynamic inductance.
91. In actual work, it is necessary to test and analyze switch oscillation signals, video signals, etc. How to do it?
A: TEK's TDS5000 series oscilloscopes can easily measure and analyze these two types of signals. For the driving signal of the switching power supply, our TDSPWR2 provides four types of analysis: duty cycle trend analysis, switching frequency trend analysis. Width and cycle trend analysis: TDS5000 oscilloscopes have more abundant video triggers, can apply multiple formats, and 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 supplies?
Answer: Although the design of the transformer is calculated theoretically, it still requires multiple tests and adjustments due to differences in the magnetic core and winding method. Generally, the primary inductance is calculated first, the core material and frame size are selected according to the output power, and then some parameters such as the core cross-sectional area 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 can we completely capture and store a one-time random signal, and then replay and analyze it?
A: If the so-called random signal being measured is a single signal, then just set the vertical and horizontal scales to match the signal, adjust the trigger level, use a single trigger to wait for the signal to appear, and then use SAVE/RECALL to save it in ref and call it out at any time; if the signal is some kind of anomaly that appears 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. Are there any special requirements for starting a switching power supply at low temperatures (e.g. below -20°C)?
A: The key is the temperature range of the device selection, such as capacitors, MOSFETs, diodes, etc.
95. Switching power supplies always have electromagnetic radiation and may be interfered by other electrical equipment. How can we avoid interference from other electrical equipment and effectively prevent the device from radiating outward?
Answer: Since the switching power supply works in a high voltage and high current switching state, the electromagnetic compatibility problems it causes 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 is mainly due to the common impedance between the interference source and the interfered object, through which the interference signal enters the interfered object. Line coupling is mainly the mutual coupling caused by parallel wiring of the conductors or PCB lines that generate interference voltage and interference current. Electric field coupling is mainly due to the coupling of the induced electric field generated by the existence of potential difference to the interfered object.
Magnetic field coupling is mainly the coupling of low-frequency magnetic field generated near high-current pulse power lines to the interference object. Electromagnetic wave coupling is mainly the coupling of high-frequency electromagnetic waves generated by pulsating voltage or current, which radiate outward through space and produce coupling to the corresponding interfered object. In fact, each coupling method cannot be strictly distinguished, but the emphasis is different. From the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility of switching power supplies, we can start from three aspects.
1) Reduce the interference signal generated by the interference source;
2) Cut off the propagation path of interference signals;
3) Enhance the anti-interference ability of the interfered object.
When solving the electromagnetic compatibility problem inside the switching power supply, the above three methods can be used in combination, with cost-effectiveness and ease of implementation as the premise. The external interference generated by the switching power supply, such as power line harmonic current, power line conducted interference, electromagnetic field radiation interference, etc., can only be solved by reducing the interference source.
On the one hand, it can enhance the design of input and output filter circuits, improve the performance of active power factor correction (APFC) circuits, reduce the voltage and current change rate of switching tubes and rectifier freewheeling diodes, and adopt various soft switching circuit topologies and control methods.
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 treatment should be performed. 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 the small energy, a combination of zinc oxide varistor and gas discharge tube can be used to solve the problem.
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, we should start from the following aspects:
Pay attention to the correct distinction between digital circuit and analog circuit PCB wiring, and the correct decoupling of digital circuit and analog circuit power supplies;
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 impact of ground loops;
When wiring, pay attention to the spacing between adjacent lines and the nature of the signal to avoid crosstalk; reduce ground impedance; reduce the area surrounded by high-voltage and high-current lines, especially the primary side of the transformer, the switch tube, and the power supply filter capacitor circuit;
Reduce the area enclosed by the output rectifier circuit, freewheeling diode circuit and 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 testing of current harmonics according to the EN61000-3-2 standard.
96. What data is obtained in SOA testing, 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 startup power-on occurs, the safe operating area may be exceeded for only a few cycles, and this is not easy to detect. The device will not be damaged by the impact, but it is also an accumulation for the device, and the device margin may not be enough.
97. How to test jitter components using an oscilloscope?
A: Deterministic jitter can be measured with an oscilloscope. The time width of the rising/falling edge can be read on the oscilloscope. It can be converted into UIp-p according to the signal period, 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?
A: 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 a repetitive signal 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. The manufacturer claims that the bandwidth of the oscilloscope can reach so many megabytes, which 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 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?
A: 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 resolution 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. Oscilloscopes need to have a place to put data after capturing a signal before capturing more signals, unless continuous waveform acquisition similar to the frame rate of television is used to store data in pixel images. This acquisition and equivalent display rate is very high, but the data format makes the amount of data required for further external analysis very large. Except for the special processing mentioned above, oscilloscopes can only continuously acquire and display signals at a very low speed.
Digitizers can achieve continuous throughput rates of 100MS/s or more, limited only by the speed of the memory bus. For example, a PCI bus digitizer card can transfer data at a rate of 100MB/s, and the PCI bus can operate at 66MS/s (132MB/s). Oscilloscope throughput is limited by the slower, lower I/O capabilities that can handle data. Slower digitizers and data loggers can write data directly to hard disk, archiving several GB of data, while oscilloscopes generally only have a maximum of 16MB. If you look at data transfer rates from another perspective, many applications only need to capture sporadic data, but these bursts may be close together. This is when fast data transfer is important. Examples of such signals are scanning radar with high repetition frequency (PRF), time-resolved ultrasonic sonar, time-of-flight mass spectrometers, and nucleon counting applications.
100. What is a combination oscilloscope?
A: 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 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, and when using a combination oscilloscope, regardless of the signal repetition rate, the brightest display can be obtained.
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