Digital Storage Oscilloscope Principles
Digital storage oscilloscopes are different from general analogue oscilloscopes in that they convert the collected analogue voltage signals into digital signals, which are analysed, processed, stored, displayed or printed by the internal microcomputer. These oscilloscopes usually have programmable and remote control capabilities, through the GpIB interface can also be transmitted to the computer and other external equipment for analysis and processing.
Its work process is generally divided into two stages of storage and display. In the storage stage, the first analog signal to be measured by the sampling and quantification, converted to digital signals by the A/D converter, sequentially stored in RAM, when the sampling frequency is high enough, you can achieve the signal without distortion storage. When the need to observe this information, as long as the appropriate frequency of this information from the memory RAM in accordance with the original order out of the D / A converter and LpE filtering sent to the oscilloscope can be observed after the restoration of the waveform.
The afterglow time of the p31 phosphor on the CRT of a normal analogue oscilloscope is less than 1 ms. In some cases, a CRT with p7 phosphor can give an afterglow time of about 300 ms. As long as a signal is illuminated by the phosphor, the CRT will continuously display the signal waveform. When the signal is removed, the sweep on the CRT with p31 material is rapidly dimmed, while the sweep on the CRT with p7 material stays a little longer.
So what if the signal is only a few times a second, or the period of the signal is only a few seconds, or even the signal is only burst once? In this case, the signals are almost, if not completely, unobservable using the analogue oscilloscopes we have described above.
The so-called digital storage is to store the signal in the oscilloscope in the form of digital code. After the signal enters the digital storage oscilloscope, or DSO, and before the signal reaches the deflection circuit of the CRT (Figure 1), the oscilloscope samples the signal voltage at regular intervals. These samples are then converted using an analogue/digital converter (ADC) to produce a binary word representing each sampled voltage. This process is called digitisation.
The binary values obtained are stored in 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 megabits per second (20 MS/s) to 200 MS/s. The data stored in the memory is used to reconstruct the signal waveform on the oscilloscope screen. Therefore, the circuitry between the input signal connector in the DSO and the oscilloscope CRT is more than just analogue circuitry. The waveforms of the input signals are stored in the memory before they are displayed on the CRT, and the waveforms we see on the oscilloscope screen are always the waveforms reconstructed from the captured data, not the direct waveforms of the signals added to the input connectors.
