Solution to the Distortion of High Frequency Pulse Signal by Oscilloscope

An engineer from Shenzhen Futian Huaqiangbei specializes in the research and development of screens. He needs to use an oscilloscope to measure a series of pulse signals when the Apple tablet ipad is powered on. After the oscilloscope captures it, he can simulate this signal against it. . But this friend failed to test several times, or was not satisfied with the captured signal, so he deliberately brought his Microsyn flat-panel oscilloscope and other related equipment to the door for consultation.

First, he demonstrated his measurement method. He needs to measure three signals in total, which are connected to the three channels of the oscilloscope. When channel 3 is powered on to generate a direct current, channel 1 and channel 2 will respectively generate a signal with pulse positive and negative intervals and different pulse widths, and what he needs to observe is the pulse change law of channel 1, and use this as a basis Out of simulation.

The direct current generated by channel 3 is at two volts, and the pulses of channel 1 and channel 2 are within ±500mV. Therefore, he set the vertical scale of channel 1 and channel 2 to 200mV/div, and the vertical scale of channel 3 to 1V/div. Then he set the time base of the oscilloscope to 500ms, that is, a 500*14ms waveform recorded on a screen, which is a 7-second signal.

Then he connects the signals to the three channels respectively, and then powers on. The oscilloscope enters the scrolling mode at 500ms time base, so he can see the signal changes in real time. After capturing a screen signal, he presses the pause button , And then adjust the time base to expand the signal, and observe the signal at the pulse-intensive part of the channel. However, he was greatly disappointed by the waveform he saw after unfolding, because the expected square wave became a sawtooth wave. Even part of the pulse signal is lost.

In fact, there is no problem with his operation. The problem is that his operation must require the oscilloscope to have a large memory depth, so that when the time base is large, the sampling rate will not decrease too much. One cycle of his pulse signal is actually about 1us, that is, the frequency of 1M. At this time, the bandwidth of the oscilloscope still meets the measurement conditions, but the sampling rate has fallen too much due to the limitation of the storage depth. The ideal measurement sampling rate should be around 5M/s-20M/s.

Here is a basic knowledge point to share, that is, the real-time sampling rate of the oscilloscope is = the storage depth of the oscilloscope ÷ the waveform recording duration. This formula shows that since the storage depth of the oscilloscope is fixed, the longer the waveform recording duration is, the real-time sampling of the oscilloscope The lower the rate. When we buy an oscilloscope, we always see that the oscilloscope is marked with a sampling rate of 1G/s or 2G/s, and we often ignore the memory depth indicator. In fact, during the measurement process, if the oscilloscope’s memory depth is too low, the oscilloscope cannot maintain The sampling rate of this label.

I found the problem and solved it easily. First, we set the storage depth of the oscilloscope to 28Mpts, which is automatic by default. Since the oscilloscope has three channels turned on, each channel is divided into 7Mpts.

Then through the overall observation of the previous captured signal, we set the time base to 1ms, set the trigger mode to edge-rising trigger, and the trigger level is moved up to 292mV, and then click Single SEQ, we plan to use a single trigger to capture the signal . After setting, power on, and then the oscilloscope will capture the signal as shown in the figure below.

Then, we stop the signal, adjust the time base and expand the signal, and then we can clearly see each pulse of channel one and the pulse with a larger pulse width. Users are more curious about why there are more obvious protrusions above the pulse signal, that is, overshoot. In fact, it is caused by the too long ground wire. Turning on the low-pass filter can also alleviate this display situation.

Later, we took a prototype that is about to be launched. The memory depth is much larger. We measured it with the method he started. Since the memory depth is large enough, there is still no distortion after the 500ms time base is expanded. Therefore, the importance of storage depth is further verified.

, the user saved the waveform data he needed and returned with a full load. I believe his problem will be solved.