Principle of Digital Storage Oscilloscope

The digital storage oscilloscope is different from the general analog oscilloscope. It converts the collected analog voltage signal into a digital signal, and the internal microcomputer performs analysis, processing, storage, display or printing. This type of oscilloscope usually has program control and remote control capabilities, and can also transmit data to external devices such as computers for analysis and processing through the GPIB interface.
The basic principle of the digital storage oscilloscope is shown in the figure. Its working process is generally divided into two stages: storage and display. In the storage stage, the analog signal to be measured is first sampled and quantized, and then converted into a digital signal by the A/D converter and stored in the RAM in sequence. When the sampling frequency is high enough, the signal can be stored without distortion. When it is necessary to observe this information, just take this information out of the memory RAM in the original order at a suitable frequency, and send it to the oscilloscope after D/A conversion and LPE filtering to observe the restored waveform. The 
afterglow time of the P31 fluorescent substance on the CRT of an ordinary analog oscilloscope is less than 1ms. In some cases, a CRT using a P7 fluorescent substance can give an afterglow time of about 300ms. As long as there is a signal irradiating the fluorescent material, the CRT will continue to display the signal waveform. When the signal is removed, the trace on the CRT using P31 material quickly darkens, while the trace on the CRT using P7 material stays a little longer.
So, what will happen if the signal only occurs a few times in one second, or the signal cycle is only a few seconds, or even the signal only occurs once in a burst? In this case, the analog oscilloscope we introduced above can hardly or even completely observe these signals.
So-called digital storage is to store signals in the form of digital coding in the oscilloscope. After the signal enters the digital storage oscilloscope, or DSO, before the signal reaches the deflection circuit of the CRT (Figure 1), the oscilloscope will sample the signal voltage at a certain time interval. Then an analog/digital converter (ADC) is used to convert these sampled values ​​to generate binary words representing each sampled voltage. This process is called digitization.
The binary values ​​obtained are stored in the memory. The rate at which the input signal is sampled is called the sampling rate. The sampling rate is controlled by the sampling clock. For general use, the sampling rate ranges from 20 megasamples per second (20MS/s) to 200MS/s. The data stored in the memory is used to reconstruct the signal waveform on the oscilloscope screen. Therefore, the circuit between the input signal connector and the oscilloscope CRT in the DSO is not just an analog circuit. The waveform of the input signal must be stored in the memory before it is displayed on the CRT. The waveform we see on the oscilloscope screen is always a waveform reconstructed from the acquired data, rather than a direct waveform display of the signal applied to the input connector.