As long as you are engaged in power supply design, you must be helpless with the oscilloscope. The oscilloscope converts electrical signals that are invisible to the naked eye into visible ripples and displays them. But 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 creates difficulties for the operation of the oscilloscope. In order to keep up with market changes, manufacturers have introduced new features, and the functions of oscilloscopes continue to increase. But for some people who are new to the oscilloscope, it is not very good to control these feature-rich instruments. This article will help you understand the oscilloscope through 10 questions. Question 1: Each oscilloscope has a frequency range, such as 10M, 60M, 100M... At present, the oscilloscope is nominally 60MHz. Can it be understood that it can measure 60MHz at the maximum? It can be measured when measuring a square wave of 4.1943MHz. No, what is the reason? A: A 60MHz bandwidth oscilloscope does not mean that a 60MHz signal can be measured very well. According to the oscilloscope bandwidth definition, if you input a 60MHz sine wave with a peak-to-peak value of 1V to a 60MHz bandwidth oscilloscope, you will see a 0.707V signal (30% amplitude measurement error) on the oscilloscope. If testing a square wave, the reference standard for the oscilloscope should be the signal rise time, the oscilloscope bandwidth = 0.35 / signal rise time × 3, and the rise time measurement error at this time is about 5.4%. The oscilloscope's probe bandwidth is also important. If the oscilloscope probe used, including its front-end accessory, has a very low system bandwidth, the oscilloscope bandwidth will be greatly reduced. If you use a 20MHz bandwidth probe, the maximum bandwidth that can be achieved is 20MHz. If the 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 influence on the square wave around 4MHz, because the speed is not very fast. Also look at the oscilloscope manual, some 60MHz oscilloscopes in the 1:1 setting, the actual bandwidth will be sharply reduced to below 6MHz, for the square wave around 4MHz, the third harmonic is 12MHz, the fifth harmonic is 20MHz If the bandwidth is reduced to 6MHz, the amplitude of the signal is greatly attenuated. Even if the signal can be seen, it is definitely not a square wave, but a sine wave whose amplitude is attenuated. Of course, there are many reasons why the signal cannot be detected. If the probe is not in good contact (this phenomenon is easy to eliminate), it is recommended to connect a function generator with a BNC cable to verify that the oscilloscope has any problems. The probe has no problem, such as If you have any questions, you can contact the manufacturer directly. Question 2: Some transient signals are slightly lost, how to capture and make them reappear? A: Set the oscilloscope to single acquisition mode (trigger mode is set to Normal, trigger condition is set to edge trigger, and the trigger level is adjusted to the appropriate value, then the scan mode is set to single mode), pay attention to the oscilloscope's memory depth. The time at which the signal is acquired and the maximum sample rate that can be used will be determined. Question 3: Cyclic jitter in a PLL can measure the quality of a design, but it is very difficult to measure accurately. Is there any method or skill? A: When using an oscilloscope, pay attention to whether its own jitter-related metrics meet the test requirements, such as the trigger jitter indicator of the oscilloscope itself. At the same time, it should be noted that when using different probes and probes to connect accessories, if the system bandwidth of the oscilloscope is not guaranteed, the measurement results will be inaccurate. In addition, the measurement of the PLL setup time can be done using the oscilloscope + USB-GPIB adapter + software option, or a cheaper modulation domain analyzer. Question 4: Why does the oscilloscope sometimes fail to capture the amplified current signal? A: If the signal does exist, but the oscilloscope can sometimes catch it and sometimes it can't be caught, this may be related to the oscilloscope's settings. Normally, the oscilloscope trigger mode can be set to Normal, the trigger condition is set to edge trigger, and the trigger level is adjusted to the appropriate value, and then the scan mode is set to single mode. If this method is not enough, it may be that the instrument has a problem. Question 5: How to measure power supply ripple? A: You can first capture the entire waveform with an oscilloscope, then zoom in on the portion of the ripple you are interested in to observe and measure (either auto-measurement or cursor measurement), and use the oscilloscope's FFT function to analyze from the frequency domain. Question 6: How is the new digital oscilloscope used for microcontroller development? Answer: The general working speed of I2C bus signal does not exceed 400Kbps. Recently, several Mbps chips have appeared. Some oscilloscopes do not need to consider the influence of different speeds when setting the trigger condition, but for other buses, such as CAN bus, they need to The current actual working speed of the CAN bus is set on the oscilloscope so that the oscilloscope can correctly understand the protocol and trigger it correctly. If you want to further analyze the Inter-IC bus signal, such as protocol-level analysis, you can use a logic analyzer, but the price is relatively high. Question 7: Questions about analog and digital oscilloscope comparisons: Which ones are more advantageous for analog and digital oscilloscopes when observing the details of a waveform (for example, observing 1% or less of parasitic waveforms at zero crossings and peaks)? Digital oscilloscopes generally provide online display The root mean square value, what is its accuracy? A: Observe the parasitic waveform below 1%. Whether it is an analog oscilloscope or a digital oscilloscope, the observation accuracy is not very good. The vertical accuracy of an analog oscilloscope is not necessarily higher than that of a digital oscilloscope. For example, the vertical accuracy of an analog oscilloscope with a 500MHz bandwidth is ±3%, which is no more advantageous than a digital oscilloscope (usually 1-2% accuracy), and for details, digital oscilloscope The automatic measurement function is more accurate than the manual measurement of an analog oscilloscope. For the accuracy measurement of the oscilloscope's amplitude, many people use A/D digits to measure. In fact, it will vary depending on the oscilloscope bandwidth you use, the actual sample rate setting, and so on. If the bandwidth is not enough, the amplitude measurement error caused by itself is very large. If the bandwidth is enough, the sampling setting is very high, and the actual amplitude measurement accuracy is not as good as when the sampling rate is low (sometimes refer to the oscilloscope user manual, it It is possible to give the oscilloscope's A/D actual effective number of bits at different sampling rates. In general, the oscilloscope measures the amplitude, including the rms value, which is often not as accurate as the multimeter. Similarly, the measurement frequency is not as good as the frequency counter. 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 ratio of 1:1. Imagine a 10M square wave, its The pulse width in the positive or negative direction is 50 ns. Under what circumstances can you really use the performance of 5 ns? A: There are two typical applications for glitch/pulse width triggering. One is synchronous circuit behavior. Pulse width triggering is an option when using it to synchronize serial signals, or for applications with very high interference that cannot be triggered with edges. The other is to find anomalies in the signal, such as narrow glitch caused by interference or competition. Since the anomaly is occasional, it must be captured with a glitch trigger (there is also a method of peak detection, but the peak detection method It may be limited by its maximum sampling rate, so it is generally only visible and not measurable). If the pulse width of the measured object is 50 ns, and the signal has no problem, that is, there is no signal distortion or narrowing caused by interference, competition, etc., the edge can be synchronized to trigger the signal without using a glitch trigger. Depending on the application, the 5 ns indicator may not be used. The general user sets the pulse width trigger to 10 ns to 30 ns. Question 9: When choosing an oscilloscope, the most common consideration is bandwidth. Under what circumstances should the sampling rate be considered? A: Depends on the object being tested. Under the premise that the bandwidth is satisfied, it is hoped that the minimum sampling interval (the reciprocal of the sampling rate) can capture the required signal details. There are some empirical formulas about sampling rate in the industry, but basically they are all derived from the oscilloscope bandwidth. In practical applications, it is better not to use an oscilloscope to measure signals of the same frequency. If the sine wave selection oscilloscope bandwidth should be more than three times the frequency of the measured sinusoidal signal during the selection, 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 sample rate is sufficient to capture the signal details. When using an oscilloscope, verify that the sample rate is sufficient by the following method. Stop the waveform and amplify the waveform. If the waveform is changed (such as some amplitude), the sampling rate is not enough. In addition, the dot display can also be used to analyze whether the sampling rate is sufficient. Question 10: How to understand "When the waveform sampling rate is insufficient, stop the waveform and amplify the waveform. If the waveform is changed (such as some amplitude), the sampling rate is not enough. Otherwise, it can be used. Analyze whether the sampling rate is sufficient."? A: I have done such an experiment. The object under test was a signal that looked very random and changed at a high speed. The user set the trigger level to about -13V. When the waveform is collected and wants to enlarge the measurement details, it is found that when the oscilloscope time base (SEC/DIV) setting is changed, the signal amplitude suddenly becomes smaller. At that time, the oscilloscope is changed to a dot display, and it seems that the number of points (storage depth) is not enough, but After comparing the dot display and the vector display, it is found that if the vector display has certain credibility, then the signal in the current two sampling intervals (the reciprocal of the sampling rate) has a sudden change, but it cannot be collected (the sampling interval is not fine enough, That is, the sampling rate is not high enough). I switched to an oscilloscope with the same depth of storage but a high sampling rate and found that the problem disappeared. Memory depth also affects the actual maximum sample rate that the oscilloscope can use. A storage depth that is too shallow can be a problem because the memory depth may limit the maximum sample rate that can actually be used, but in essence the sample rate is insufficient and the signal details are lost. The storage depth is not deep enough, which may result in an actual sampling rate that is not high. This has little to do with the specifications provided by the manufacturer. Through these 10 questions and answers, I hope that some of the novices can answer the oscilloscope questions and help them get started quickly. Kitchen Mat Set,Foam Floor Mat,Cushioned Kitchen Mat,Environmental Friendly Kitchen Mat Fuzhou Will Trade Import and Export Co., Ltd. , https://www.will-trade.com
October 14, 2021