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If you are engaged in power supply design, you cannot do without the help of an oscilloscope. An oscilloscope can convert invisible electrical signals into visible waveforms and display them. However, with the rapid advancement of technology, the electrical signals in circuit systems are becoming faster and the rise time shorter. This poses challenges for the oscilloscope’s operation. To keep up with market changes, manufacturers continually innovate, and the functions of oscilloscopes are constantly increasing. However, for those who are new to oscilloscopes, it may be difficult to master these feature-rich instruments. This article will help everyone understand oscilloscopes through 10 questions.
Question 1: Every oscilloscope has a frequency range, such as 10M, 60M, 100M… Currently, an oscilloscope is rated at 60MHz. Can it be understood that it can measure up to 60MHz? If a square wave of 4.1943MHz cannot be measured, what is the reason?
Answer: A 60MHz bandwidth oscilloscope does not necessarily mean it can accurately measure a 60MHz signal. According to the definition of oscilloscope bandwidth, if a 1V peak-to-peak 60MHz sine wave is input to a 60MHz bandwidth oscilloscope, a 0.707V signal will be seen on the oscilloscope (30% amplitude measurement error). If testing a square wave, the reference standard for the oscilloscope should be the signal rise time, where oscilloscope bandwidth = 0.35 / signal rise time × 3, resulting in a rise time measurement error of about 5.4%.
The bandwidth of the oscilloscope probe is also important. If the bandwidth of the probe used, including its front-end attachments, is low, it will greatly reduce the oscilloscope’s bandwidth. For example, using a 20MHz bandwidth probe will limit the maximum bandwidth to 20MHz. If a connecting wire is used at the probe’s front end, it will further degrade the probe’s performance, but it should not significantly affect a square wave around 4MHz, as the speed is not very high.
Additionally, check the oscilloscope’s manual; some 60MHz oscilloscopes may have their actual bandwidth sharply reduced to below 6MHz at a 1:1 setting. For a square wave around 4MHz, its third harmonic is 12MHz and its fifth harmonic is 20MHz. If the bandwidth drops to 6MHz, there will be significant amplitude attenuation, and even if the signal is visible, it will not be a square wave but a sine wave with reduced amplitude.
Of course, there may be various reasons for not being able to measure the signal, such as poor probe contact (this phenomenon is easy to rule out). It is recommended to connect a function generator to the oscilloscope using a BNC cable to check if the oscilloscope itself has a problem or if the probe has an issue. If there are problems, you can contact the manufacturer directly.
Question 2: How to capture and reproduce some transient signals that are fleeting?
Answer: Set the oscilloscope to single acquisition mode (set the trigger mode to Normal, trigger condition to edge-triggered, and adjust the trigger level to an appropriate value, then set the scan mode to single). Note that the oscilloscope’s storage depth will determine the time for capturing signals and the maximum sampling rate that can be utilized.
Question 3: In a PLL, cycle jitter can measure the quality of a design, but it is very difficult to measure accurately. Are there any methods or tips?
Answer: When using an oscilloscope, pay attention to whether the jitter-related indicators of the oscilloscope itself meet the testing requirements, such as the oscilloscope’s own trigger jitter specification. Also, be careful when using different probes and probe connection accessories, as failing to ensure the oscilloscope’s system bandwidth may lead to inaccurate measurement results. For measuring PLL setup time, you can use an oscilloscope + USB-GPIB adapter + software options, or a relatively inexpensive modulation domain analyzer.
Question 4: Why can’t the oscilloscope sometimes capture the amplified current signal?
Answer: If the signal indeed exists but the oscilloscope sometimes captures it and sometimes does not, it may be related to the oscilloscope’s settings. Typically, you can set the oscilloscope trigger mode to Normal, trigger condition to edge-triggered, and adjust the trigger level to an appropriate value, then set the scan mode to single. If this approach still does not work, it may indicate a problem with the instrument.
Question 5: How to measure power supply ripple?
Answer: You can first use the oscilloscope to capture the entire waveform, then zoom in on the ripple portion of interest to observe and measure (either automatic measurement or cursor measurement is acceptable), and also utilize the oscilloscope’s FFT function for frequency domain analysis.
Question 6: How can new digital oscilloscopes be used in microcontroller development?
Answer: I2C bus signals generally operate at a rate not exceeding 400Kbps, and recently some chips have appeared that operate at several Mbps. Some oscilloscopes do not require consideration of different rates when setting trigger conditions, but for other buses, such as CAN bus, it is necessary to first set the current actual operating rate of the CAN bus on the oscilloscope so that it can correctly understand the protocol and trigger correctly. If you want to perform further analysis of Inter-IC bus signals, such as protocol-level analysis, a logic analyzer can be used, but it is relatively expensive.
Question 7: Regarding the comparison between analog and digital oscilloscopes: which has more advantages in observing the details of waveforms (for example, when crossing zero and at peaks, observing parasitic waveforms below 1%)? Digital oscilloscopes generally provide online RMS value display; what is its accuracy?
Answer: When observing parasitic waveforms below 1%, neither analog nor digital oscilloscopes have great observational accuracy. The vertical accuracy of an analog oscilloscope may not necessarily be higher than that of a digital oscilloscope. For example, a 500MHz bandwidth analog oscilloscope may have a vertical accuracy of ±3%, which is not better than that of a digital oscilloscope (which typically has an accuracy of 1-2%). Furthermore, for detail measurement, the automatic measurement function of a digital oscilloscope is generally more precise than manual measurement on an analog oscilloscope.
For the measurement accuracy of the oscilloscope’s amplitude, many people evaluate it based on the A/D bit count. In reality, this can vary based on the bandwidth of the oscilloscope used and the actual sampling rate settings. If the bandwidth is insufficient, the amplitude measurement error will be significant. If the bandwidth is sufficient and the sampling settings are high, the actual amplitude measurement accuracy may be lower than when the sampling rate is low (sometimes you can refer to the oscilloscope’s user manual, which may provide the effective number of bits for the A/D at different sampling rates). Overall, the measurement accuracy of oscilloscopes for amplitude, including RMS values, is often not as good as that of a multimeter; similarly, for frequency measurement, they are not as accurate as frequency counters.
Question 8: What is the significance of the glitch trigger indicator? If there is a 100MHz oscilloscope and the measured square wave signal is around 10M, with a duty cycle of about 1:1, imagine a 10M square wave with a positive or negative pulse width of 50ns; under what circumstances can the 5ns performance truly be utilized?
Answer: Glitch/pulse width triggering generally has two typical applications. One is for synchronizing circuit behavior. For example, using it to synchronize serial signals or when interference is severe and edge-triggering cannot correctly synchronize the signals, pulse width triggering becomes a choice. The other application is to discover anomalies in signals, such as narrow glitches caused by interference or contention. Since these anomalies are sporadic, they must be captured using glitch triggering (another method is peak detection, but peak detection may be limited by its maximum sampling rate, so generally, it can only be viewed and not measured). If the pulse width of the measured object is 50ns and the signal is normal, meaning there are no issues like interference or contention causing signal distortion or narrowing, then edge triggering can synchronize that signal without using glitch triggering. Depending on the application, the 5ns specification may not necessarily be utilized; typically, users set the pulse width trigger to 10ns-30ns.
Question 9: When selecting an oscilloscope, bandwidth is generally the main consideration. Under what circumstances should sampling rate be taken into account?
Answer: It depends on the object being measured. Given that the bandwidth is sufficient, the minimum sampling interval (the inverse of the sampling rate) should be able to capture the necessary signal details. There are some empirical formulas regarding sampling rates in the industry, but they are generally derived based on oscilloscope bandwidth. In practical applications, it is best not to use the oscilloscope to measure signals of the same frequency. When selecting, for sine waves, the oscilloscope bandwidth should be more than three times the frequency of the sine wave being measured, and the sampling rate should be 4 to 5 times the bandwidth, which means actually 12 to 15 times the signal frequency. For other waveforms, it is necessary to ensure that the sampling rate is sufficient to capture the signal details. When using an oscilloscope, you can verify whether the sampling rate is adequate by stopping the waveform and zooming in; if the waveform changes (such as certain amplitudes), it indicates that the sampling rate is not enough; otherwise, there is no issue. Additionally, you can use point display to analyze whether the sampling rate is sufficient.
Question 10: How to understand the statement, “When assessing whether the waveform sampling rate is sufficient, stop the waveform and zoom in. If you find that the waveform changes (such as certain amplitudes), it indicates that the sampling rate is not sufficient; otherwise, there is no issue. Point display can also be used to analyze whether the sampling rate is sufficient.”?
Answer: An experiment was once conducted where the object being measured was a signal that appeared very random and changed rapidly, and the user set the trigger level around -13V. After capturing the waveform, when trying to zoom in for detail measurement, it was found that when changing the oscilloscope time base (SEC/DIV) setting, the signal amplitude suddenly decreased. When the oscilloscope was switched to point display, it seemed that the number of points (storage depth) was insufficient, but upon comparing point display and vector display, it was found that if the vector display had some reliability, it indicated that there was a sudden change in the signal between the current two sampling intervals (the inverse of the sampling rate) that had not been captured (the sampling interval was not fine enough, meaning the sampling rate was not high enough). I replaced it with another oscilloscope with the same storage depth but a higher sampling rate, and the problem disappeared.
Storage depth also affects the actual maximum sampling rate that can be utilized by the oscilloscope. Insufficient storage depth may be a problem because it can limit the maximum sampling rate that can actually be used, resulting in loss of signal details. If the storage depth is not deep enough, it may lead to a low actual sampling rate, which is not significantly related to the indicators provided by the manufacturer.
Through these 10 questions and answers, I hope to help some beginners resolve their doubts about oscilloscopes and assist them in getting started quickly.
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