The secret to improving your oscilloscope measurement quality 1000 times

The sampling rate is affected by the horizontal scale of the oscilloscope. The formula is:

Sampling rate = storage depth / acquisition time length

Memory depth is a constant value, acquisition time length (or trace length) is a variable that is determined by your time per division setting. As the time/division setting increases, the acquisition time length increases. Since this all has to fit within the scope’s memory depth, at some point the scope’s ADC will have to reduce its sample rate. What does this mean in practice? Let’s take the example of a frequency measurement of a 100 kHz square wave. We know the frequency is 100 kHz and is very stable, so we can use the standard deviation of the measurement results to judge the quality of the measurement. Figure 1 The horizontal display scale of the 100 kHz square wave is set to full scale, 20 milliseconds. Also, the scope’s sample rate has automatically been reduced from 5 GSa/sec to 100 MSa/sec to allow the entire trace to fit into the scope’s memory. After about 1500 measurements, the standard deviation of the measurement is 1.49 kHz (about 1.5%).

However, let’s see what happens if we select a smaller time/div setting, effectively shortening the acquisition time and increasing the sample rate. Figure 2 shows the same signal, but with the horizontal scale set to 1.2 microseconds/div. The standard deviation is now 1.5 Hz, one thousandth of what we measured previously.

The text in the picture is in Chinese and English

The text in the picture is in Chinese and English

All that changes is the horizontal scale of the signal and the sampling rate of the oscilloscope. Therefore, choosing the appropriate oscilloscope horizontal scale has a great impact on the quality of time-correlated measurements.

Just as the horizontal scale affects time-related measurements, the vertical scale also affects vertical-related measurements (voltage peak-to-peak, RMS, etc.). Let’s take the same 100 kHz square wave as an example again and look at the peak-to-peak voltage. The signal in Figure 3 is scaled to 770 mV/div. The standard deviation of the peak-to-peak measurement is 18 mV. Lowering the volts/div setting of the oscilloscope to 66 mV/div changes the standard deviation of the measurement to 1.22 mV. That’s almost a 15x improvement!

The text in the picture is in Chinese and English

Why does the vertical scale setting matter? By setting the signal scale so that it fills the screen as much as possible, we can take full advantage of the oscilloscope's resolution. Resolution is a measure of how accurate the ADC can be. The higher the resolution, the greater the number of vertical levels the ADC can detect. For example, the figure below shows a 2-bit ADC. The red sine wave is the analog input to the ADC, and the blue waveform is the digitized input. You can see that four different quantization levels are present.

This figure shows the same analog waveform digitized by a 3-bit ADC. The greater the number of quantization levels, the closer the ADC's digital output will be to the analog input.

The text in the picture is in Chinese and English

If you vertically scale your signal to fill only a portion of the oscilloscope screen, you are not actually taking full advantage of the ADC's resolution. For example, if you scale your signal to take up half of a 3-bit ADC's screen, you are left with two unused quantization levels above and below the signal. This means that your 3-bit ADC can only use four quantization levels, equivalent to the accuracy of a 2-bit ADC.

了解如何正确地设置示波器信号显示刻度，能极大地改善您的测量质量。适当的水平定标会显著改善时间相关测量的质量，而适当的垂直定标则会对垂直相关测量产生积极影响。下一次使用示波器时，请记住：正确设置信号刻度，可以获得最佳的测量结果！