Ten questions and ten answers to help you understand the use of oscilloscopes

　　As long as you are engaged in power supply design, you must use the help of an oscilloscope. An oscilloscope can convert invisible electrical signals into visible ripples and display them. However, with the rapid development of technology, the electrical signals in the circuit system are getting faster and faster, and the rise time is getting shorter and shorter. This makes it difficult for the oscilloscope to work. In order to keep up with market changes, manufacturers are constantly innovating and increasing the functions of oscilloscopes. However, for some people who are new to oscilloscopes, they cannot master these feature-rich instruments well. This article will help you understand oscilloscopes through 10 questions.

　　
　　Question 1: Each oscilloscope has a frequency range, such as 10M, 60M, 100M..., the current oscilloscope is nominally 60MHz, can it be understood that it can measure up to 60MHz? It can measure 4.1943MHz square wave can not be measured, what is the reason?
　　
　　Answer: 60MHz bandwidth oscilloscope does not mean that it can measure 60MHz signal well. According to the definition of oscilloscope bandwidth, if a 60MHz sine wave with a peak-to-peak value of 1V is input to a 60MHz bandwidth oscilloscope, a 0.707V signal will be seen on the oscilloscope (30% amplitude measurement error). If testing square waves, the reference standard for selecting an oscilloscope should be the signal rise time, oscilloscope bandwidth = 0.35/signal rise time × 3, and the rise time measurement error at this time is about 5.4%. The
　　
　　probe bandwidth of the oscilloscope is also very important. If the system bandwidth of the oscilloscope probe used, including its front-end accessories, is very low, the oscilloscope bandwidth will be greatly reduced. If a probe with a bandwidth of 20MHz is used, the maximum bandwidth that can be achieved is 20MHz. If a connecting wire is used at the front end of the probe, the performance of the probe will be further reduced, but it should not have much impact on square waves of about 4MHz because the speed is not very fast.
　　
　　In addition, you should also look at the oscilloscope manual. Some 60MHz oscilloscopes will have a sharp reduction in actual bandwidth to below 6MHz under a 1:1 setting. For a square wave of about 4MHz, 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 the signal can be seen, 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 contact of the probe (this phenomenon is easy to eliminate). It is recommended to connect a function generator with a BNC cable to check whether there is any problem with the oscilloscope itself and the probe. If there is any problem, you can contact the manufacturer directly.
　　
　　Question 2: Some transient signals are lost in a flash. How to capture and reproduce them?
　　
　　Answer: Set the oscilloscope to single acquisition mode (set the 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). Note that the storage depth of the oscilloscope will determine the time to acquire the signal and the maximum sampling rate that can be used.
　　
　　Question 3: In PLL, period jitter can measure the quality of a design, but it is very difficult to measure it accurately. Are there any methods and techniques?
　　
　　Answer: When using an oscilloscope, pay attention to whether its own jitter-related indicators meet the test requirements, such as the trigger jitter indicators of the oscilloscope itself. At the same time, pay attention to the use of different probes and probe connection accessories. If the system bandwidth of the oscilloscope cannot be guaranteed, the measurement results will also be inaccurate. In addition, the measurement of PLL setup time can be completed using an oscilloscope + USB-GPIB adapter + software options, or a relatively cheap modulation domain analyzer can be used.
　　
　　Question 4: Why can't the oscilloscope sometimes capture the amplified current signal?
　　
　　Answer: If the signal does exist, but the oscilloscope can sometimes capture it and sometimes not, this may be related to the settings of the oscilloscope. Usually, the oscilloscope trigger mode can be set to Normal, the trigger condition can be set to edge trigger, and the trigger level can be adjusted to an appropriate value, and then the scanning mode can be set to single mode. If this method still doesn't work, there may be something wrong with the instrument.
　　
　　Question 5: How to measure power supply ripple?
　　
　　Answer: You can first use an oscilloscope to capture the entire waveform, and then zoom in on the ripple part of interest to observe and measure (automatic measurement or cursor measurement is possible), and at the same time use the oscilloscope's FFT function to analyze from the frequency domain.
　　
　　Question 6: How can the new digital oscilloscope be used for microcontroller development?
　　
　　Answer: The I2C bus signal generally works at a rate of no more than 400Kbps. Recently, chips with a rate of several Mbps have also appeared. Some oscilloscopes do not need to consider the impact of different rates when setting trigger conditions. However, for other buses, such as the CAN bus, it is necessary to first set the actual working rate of the CAN bus on the oscilloscope so that the oscilloscope can correctly understand the protocol and trigger correctly. If you want to further analyze the Inter-IC bus signal, such as protocol-level analysis, you can use a logic analyzer, but it is relatively expensive.
　　
　　Question 7: Regarding the comparison between analog and digital oscilloscopes: Which one has more advantages in observing the details of the waveform (for example, observing parasitic waveforms below 1% at zero crossings and peaks)? Digital oscilloscopes generally provide online display of the root mean square value. What is its accuracy?
　　
　　Answer: When observing parasitic waveforms below 1%, both analog and digital oscilloscopes have poor observation accuracy. The vertical accuracy of an analog oscilloscope is not necessarily higher than that of a digital oscilloscope. For example, the vertical accuracy of an analog oscilloscope with a bandwidth of 500MHz is ±3%, which is not more advantageous than 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 for details.
　　
　　Many people use the number of A/D bits to measure the amplitude measurement accuracy of an oscilloscope. In fact, it will change with the bandwidth of the oscilloscope you use, the actual sampling rate setting, etc. If the bandwidth is not enough, the amplitude measurement error caused by it will be very large. If the bandwidth is enough and the sampling setting is very high, the actual amplitude measurement accuracy is not as good as the accuracy when the sampling rate is low (sometimes you can refer to the user manual of the oscilloscope, which may give the actual effective number of bits of the oscilloscope's A/D at different sampling rates). In general, the accuracy of oscilloscopes in measuring amplitudes, including the root mean square value, is often not as good as that of multimeters. Similarly, it is not as good as a frequency counter in measuring frequency.
　　
　　Question 8: What is the significance of the glitch trigger indicator? If there is a 100MHz oscilloscope, the measured square wave signal is about 10M, and it is a square wave with a duty cycle of about 1:1. Imagine that a 10M square wave has a positive or negative pulse width of 50ns. In what circumstances can the 5ns performance be truly used?
　　
　　Answer: There are generally two typical applications for glitch/pulse width triggering. One is the synchronization of circuit behavior. For example, when it is used to synchronize serial signals, or when edge triggering cannot be used to correctly synchronize signals for applications with very serious interference, pulse width triggering is an option. The other is to discover abnormal phenomena in the signal, such as narrow glitches caused by interference or competition. Since the abnormality appears occasionally, it must be captured by glitch triggering (there is also a peak detection method, but the peak detection method may be limited by its maximum sampling rate, so it is generally only visible but not measured). If the pulse width of the object under test is 50ns, and there is no problem with the signal, that is, there is no signal distortion or narrowing caused by interference, competition, etc., then the edge trigger can be used to synchronize the signal without using glitch trigger. Depending on the application, the 5ns indicator may not be used. Generally, users set the pulse width trigger to 10ns~30ns.
　　
　　Question 9: When choosing an oscilloscope, the bandwidth is generally considered the most. So under what circumstances should the sampling rate be considered?
　　
　　Answer: It depends on the object under test. Under the premise of satisfying the bandwidth, it is hoped that the minimum sampling interval (the inverse of the sampling rate) can capture the required signal details. 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 the bandwidth of the oscilloscope selected for the sine wave is more than 3 times the frequency of the sine signal to be 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 the signal details. When using an oscilloscope, you can verify whether the sampling rate is sufficient by the following method. Stop the waveform and zoom in on it. If you find that the waveform has changed (such as some amplitudes), it means that the sampling rate is not enough. Otherwise, it is fine. You can also use point display to analyze whether the sampling rate is sufficient.
　　
　　Question 10: How do you understand "When testing whether the waveform sampling rate is sufficient, stop the waveform and zoom in on it. If you find that the waveform has changed (such as certain amplitudes), it means that the sampling rate is not enough. Otherwise, it is fine. You can also use point display to analyze whether the sampling rate is sufficient."?
　　
　　Answer: I have done such an experiment. At that time, the object to be measured was a signal that looked very random and changed at a high speed. The user set the trigger level to about -13V. After the waveform was collected, I wanted to zoom in on the measurement details, but I found that when the oscilloscope time base (SEC/DIV) setting was changed, the signal amplitude suddenly became smaller. At that time, I changed the oscilloscope to point display and found that it seemed that the number of points (storage depth) was not enough. However, after comparing the point display and the vector display, I found that if the vector display has a certain degree of credibility, then the signal has a sudden change in the current two sampling intervals (the inverse of the sampling rate), but it has not been collected (the sampling interval is not fine enough, that is, the sampling rate is not high enough). I changed to an oscilloscope with the same memory depth but a higher sampling rate, and found that the problem disappeared.
　　
　　The memory depth will also affect the actual maximum sampling rate that the oscilloscope can use. Too shallow a memory depth may be a problem, because the memory depth may limit the maximum sampling rate that can be used, but in fact the sampling rate is not enough and the signal details are lost. If the memory depth is not deep enough, the actual sampling rate may not be high, which has little to do with the indicators provided by the manufacturer.
　　
　　Through these 10 questions and answers, I hope to answer some of the questions of novices about oscilloscopes and help them get started quickly.