The waveform refresh rate problem caused by the comparison of two signals with the same frequency

　　A few days ago, Zhiyuan Electronics WeChat official account launched an article called "God Has Something to Say". It was written by Mr. Xu from Qingdao Aoban Measuring Instruments! He originally wanted to use the ZDS2022 oscilloscope to test and compare two signals of the same frequency, but during the test, he found another problem. What was the problem? And why did he feel so moved that he wanted to write an article himself? What is going on? This video will reveal it to you.

　　He originally wanted to use the ZDS2022 oscilloscope to test and compare two signals of the same frequency. However, during the test, he found that only one signal was stably displayed, while the other signal could not see the waveform. So Engineer Xu was very confused. What was going on? So he asked in our newly created WeChat communication group. After the professional answer from the R&D colleagues in the WeChat group, we learned that the oscilloscope can only select one signal for triggering when working. If the phases of the two signals are irrelevant and one is unstable relative to the other, the phenomenon described by Engineer Xu will occur.

　　In order to find out the truth, Mr. Xu told us on WeChat that he had found an oscilloscope from T Company for comparative measurement. Since we did not have the DPO2024B oscilloscope that Mr. Xu used, we used the MSO4054 oscilloscope for demonstration. Mr. Xu's test object was two 10M crystal oscillators, and the signal we input was the 27M crystal oscillator signal on the ZDS2022 oscilloscope demonstration board.

　　In fact, for high-frequency signals, basic operation settings are the prerequisite for correct measurement. First, the attenuation ratio of the probe should be set to X10, and of course the oscilloscope probe ratio should also be set to 10X accordingly, so that the measurement display results on the oscilloscope screen are correct. Since the frequency of the measured signal is 27M at this time, the bandwidth limit needs to be turned off. In order to improve signal fidelity, the grounding spring that comes standard with the ZDS2022 oscilloscope probe should be used instead of the grounding alligator clip for grounding nearby.

　　The input signals of the two oscilloscopes are the same, so why are the waveforms displayed on the two oscilloscope screens different? This is obviously related to the instrument. The waveform displayed by the ZDS2022 oscilloscope is obviously more delicate. What's going on?

　　One very important indicator of an oscilloscope is the waveform refresh rate. The waveform refresh rate refers to the number of times the waveform is refreshed per second. The ZDS2022 oscilloscope has a refresh rate of up to 330,000 times per second. The refresh rate directly determines the oscilloscope's ability to capture tiny, abnormal signals. In fact, the process of the oscilloscope from acquiring signals to displaying signal waveforms on the screen is composed of several capture cycles. A capture cycle includes sampling time and dead time. The analog signal is converted into a digital signal through ADC sampling and quantization and stored at the same time. The time of the entire sampling and storage process is called the sampling time. The oscilloscope must perform measurement, calculation, display and other processing on the stored data before it can start the next sampling. This period of time is called the dead time. During the dead time, the oscilloscope does not perform waveform acquisition. After a capture cycle is completed, it will enter the next capture cycle.

　　The reciprocal of the capture cycle is the waveform refresh rate, which is 1/(sampling time + dead time). The sampling time is determined by the sampling window of the oscilloscope screen, that is, the sampling time is fixed. The dead time is determined by the processing power of the oscilloscope. For an oscilloscope with a very low refresh rate, the dead time is very large, the probability of capturing an abnormal waveform will be reduced, and the number of waveform refreshes per second is very low. Therefore, the waveform lines will be very obvious. The ZDS2022 oscilloscope has a high refresh rate, and the waveform lines will overlap, so it looks more delicate.