100 Questions on Oscilloscope Basics (Part 1)

　　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 general oscilloscope probe? Does the probe 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 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 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 - BH curve analysis, which can reflect the working state 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 use an oscilloscope to identify noise?

　　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 an 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 functions, 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 trigger 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?

　　Answer: The power frequency input may be a low frequency of 50Hz/60Hz, and the carrier 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. [page]

　　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?

　　Answer: There are several factors to consider when an oscilloscope can test noise signals: ① The amplitude of the signal being tested, whether it is a small signal. The oscilloscope can test uA-level signals with a probe. ② The frequency of the signal being tested. 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 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 at the same point each time, so the display can be stable. In addition, trigger holdoff should also be used for amplitude modulation signals.

　　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 get 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?

　　A: There are two types of USB signal testing: The first type 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 stamped. 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 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 signal's packet fidelity 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 a similar equivalent sampling method. 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 less than 3%. For rise time measurements of less than 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 Agilent'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?

　　A: 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. [page]

　　27. How to measure the phase noise in a design that has very high requirements on the phase noise parameters of the clock?

　　A: 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-choice-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 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 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. There is no better problem. This problem may usually be that the signal is complex and there are many points that meet the triggering conditions. It is impossible to trigger at the same position every time to obtain a stable display. At this time, an external trigger is needed. An example is as follows:

　　Observe the signal above. Since each point ABCD will be triggered, the waveform displayed by the oscilloscope will not be stable. At this time, you can use the following signal as the 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, 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?

　　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) = 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?

　　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).