How to measure the waveform capture rate of an oscilloscope

Some friends who bought an oscilloscope may be curious about the refresh rate of the oscilloscope and wonder if they can measure it. Compared with the sampling rate, storage depth and other indicators determined by hardware characteristics, the refresh rate is completely determined by the processor processing method. Reasonable data processing methods can get a higher refresh rate. Next, we will teach you how to measure the refresh rate of the oscilloscope. Interested friends can pick up the oscilloscope in their hands to measure it.

First, let us understand the concept of oscilloscope refresh rate (also called waveform capture rate).

Waveform capture rate concept

What is the concept of waveform capture rate? The waveform capture rate is relative to the digital oscilloscope. It takes time for the digital oscilloscope to sample, process data, and display the data on the screen. We can also understand it this way, that the oscilloscope will blink. They will open their eyes several times per second to capture the signal, and close their eyes to process the data and display the data on the screen.

The time between processing data and displaying it on the screen is called dead time.

The oscilloscope does not sample data during the dead time and cannot detect changes in the signal. Therefore, we cannot actually see all waveforms on the screen. The waveforms we see are actually divided into sections by the dead time. Hence the term waveform capture rate.

Some people may be curious. The screen refresh rate of an oscilloscope is generally only tens of Hz, and it can only refresh dozens of pictures per second. Can the screen display so many waveforms? In fact, there is a waveform synthesizer in the oscilloscope. As shown in the figure below, the waveform synthesizer will synthesize multiple frames of waveforms into one picture. Although the screen only refreshes dozens of pictures per second, each picture contains tens of thousands of frames of waveforms, so the screen can display the waveforms at a higher refresh rate.

Preparing tools

1. Oscilloscope under test

2. Measurement oscilloscope

3. Signal generator

4. 2 BNC to BNC cables

Every time the oscilloscope samples a waveform, it will output a pulse signal in Aux out. This Aux out interface is usually connected together with other BNC channels of the oscilloscope. Therefore, we can measure the waveform capture rate of the oscilloscope by measuring the Aux out signal. This time we want to measure the waveform capture rate of the Macosim STO1104C oscilloscope.

We use a BNC to BNC cable to input the signal generator into a channel of the oscilloscope under test, and use another BNC to BNC cable to connect the Aux out interface of the oscilloscope under test and a channel of the measuring oscilloscope.

Oscilloscope settings under test

The nominal waveform refresh rate of an oscilloscope is usually the maximum value, but in fact the waveform capture rate is inconsistent under each setting and each horizontal time base gear. We need to find the setting with the maximum waveform capture rate. First, we set the signal generator to generate a 2MHz sine wave input to the oscilloscope under test, then set the sampling mode of the oscilloscope under test to normal, the afterglow to automatic, the record length to automatic, adjust the time base to 50ns, turn on the frequency meter of channel 1 of the measuring oscilloscope, and the reading is about 80KHz. It can be obtained that the maximum waveform capture rate of the oscilloscope under test is 80,000 times per second.