What does the sampling rate of an oscilloscope mean?

Imagine how a photo can be clear? Of course, the more pixels there are, the closer the original information contained in the photo is to reality, and the clearer it looks.

The waveform we see on an oscilloscope can actually be understood as a photo. The more points the photo contains, the closer it is to the real thing. The storage depth of an oscilloscope expresses how many data points the oscilloscope can store at most. For example, a storage depth of 28Mpts means that the oscilloscope can store up to 28 million sampling points.

For taking a still photo, the speed of the camera doesn't matter much, because the result will not change. But because the signal is constantly changing, it is more like taking a moving photo for the oscilloscope, and it is a very high-speed movement. At this time, in addition to the number of sampling points, the speed of sampling point acquisition is also crucial. The oscilloscope's reconstruction of a signal depends not only on how many data points there are, but also on the speed of collecting data points. The sampling rate of an oscilloscope is the ability of the oscilloscope to collect how many data points per second. If the sampling rate of the oscilloscope is insufficient, then we cannot accurately see the true appearance of the signal.

The signal input to the oscilloscope also changes continuously in both the time axis and the voltage axis. Since computers can only process discrete digital signals, such signals cannot be described and processed digitally. Therefore, a high-speed ADC is needed to sample and quantize the signal, which is the process of digitization. After analog-to-digital conversion, the waveform that changes continuously in time and voltage becomes a series of continuously changing digital sampling points.

In the process of sampling or digital quantization, if you want to reconstruct the waveform as realistically as possible, the most critical issue is whether the sampling points on the time axis are dense enough and the quantization level of the voltage in the vertical direction. The interval of the sampling points in the horizontal direction depends on the sampling rate of the oscilloscope's ADC, while the quantization level of the voltage in the vertical direction depends on the number of bits of the ADC.

The operation process of an oscilloscope is roughly like this:

We input a signal to the oscilloscope through the probe. After the measured signal passes through the amplification, attenuation and other signal conditioning circuits at the front end of the oscilloscope, the high-speed ADC analog-to-digital converter performs signal sampling and digital quantization. The sampling rate of the oscilloscope is the frequency of the sampling clock when the input signal is converted to analog-to-digital. In layman's terms, it is the sampling interval, and one sampling point is collected at each sampling interval. For example, a sampling rate of 1GSa/s means that the oscilloscope has the ability to collect 1 billion sampling points per second, and its sampling interval is 10 nanoseconds.

For real-time oscilloscopes, real-time sampling is currently widely used. Real-time sampling means that the waveform signal being measured is sampled continuously at high speed at equal intervals, and then the waveform is reconstructed or restored based on these continuously sampled points. In the real-time sampling process, a key point is to ensure that the sampling rate of the oscilloscope is much faster than the change of the measured signal.

So how much faster should it be? You can refer to the Nyquist theorem in digital signal processing. The Nyquist theorem states that if the bandwidth of the measured signal is limited, then when sampling and quantizing the signal, if the sampling rate is more than twice the bandwidth of the measured signal, the information carried in the signal can be completely reconstructed or restored without aliasing.

The following figure shows signal aliasing caused by insufficient sampling rate. It can be seen that the frequency of the collected signal is much smaller than the original signal.

Most oscilloscopes provide several sampling modes for users to choose from. Common ones include normal sampling, average sampling, peak sampling, and envelope sampling.

In normal sampling mode, the oscilloscope samples the signal at equal time intervals to reconstruct the waveform. This mode can produce the best display effect for most waveforms.

In Peak Sampling mode, when the horizontal time base is set low, the minimum and maximum sample values ​​are retained to capture rare and narrow events (while amplifying any noise). This mode will display all pulses that are at least as wide as the sampling period. Peak Sampling mode can be used to more easily view glitches or narrow pulses. In Peak Sampling mode, narrow glitches and transition edges are displayed brighter than in "normal" sampling mode, making them easier to see. Applying Peak Sampling can avoid signal aliasing, but it will also show more actual noise.

Use the average sampling mode to average multiple acquisition results to reduce random or irrelevant noise in the displayed signal. Averaging multiple acquisition results requires a stable trigger. The number of averages can be set in the selection box after the average sampling mode. The higher the average number, the slower the displayed waveform responds to waveform changes. A compromise must be made between the speed of the waveform's response to changes and the degree of reduction in the displayed noise on the signal.

Using the envelope sampling mode, you can see the superposition effect of waveforms sampled several times, capture the maximum and minimum values ​​of a signal in the specified N sampling data, and set the number of waveform superpositions. The following figure shows an amplitude modulated signal with a waveform superposition of 32 times in envelope sampling mode.

No matter which sampling method you choose, remember to ensure that the sampling rate is at least twice the bandwidth of the signal being measured. In fact, we recommend 3-5 times or more, so that it is easier to capture abnormal information of the waveform.

The last thing worth noting is that the sampling rate of an oscilloscope is different from the bandwidth of the oscilloscope. When you open multiple channels, the sampling rate will be evenly distributed among each channel. Therefore, if you open multiple channels, be sure to double-check whether the sampling rate still meets the requirements.