Understand the working principle of oscilloscope in 5 minutes

　　The oscilloscope is the right-hand man of our electronic engineers. Playing with the oscilloscope well is one of our necessary skills. The ancients have long taught us not only to know what it is, but also why it is. Therefore, it is not enough to just play it. It is even more important to understand the working principle of the oscilloscope in order to use it better. If you still don't understand how the oscilloscope works, follow the editor to learn it together~

　　First of all, oscilloscopes are divided into two types: analog oscilloscopes and digital oscilloscopes based on their design principles. I believe everyone knows this. The earliest oscilloscopes were analog oscilloscopes, but now due to bandwidth and other issues, analog oscilloscopes have gradually been eliminated. So what is the principle of an analog oscilloscope? The following picture can explain it well:

　　The analog oscilloscope generates a periodic sawtooth signal inside to control the horizontal deflection of the silver flat electron gun, and the measured voltage signal is amplified to control the vertical deflection of the phosphor screen electron gun. In this way, the light spot or bright line is clearly displayed on the phosphor screen, which is the waveform.

　　Isn't it simple? From the perspective of design concept, analog oscilloscopes have many incomparable advantages, such as no signal waveform loss, no dead time, etc. Although digital oscilloscopes have these problems at the beginning, with the vigorous development of electronic technology, this defect has become smaller and smaller. So what is the design principle of digital oscilloscopes?

　　The waveform first passes through the probe, is amplified by the front-end amplifier, then converted by the analog-to-digital conversion unit, stored in the acquisition memory, and then displayed on the monitor.

　　In this whole process, you can easily find that the waveform is not displayed on the screen in real time, but is displayed on the waveform after being acquired in memory. Therefore, if the entire sampling and conversion time is long, a large dead time will be generated, so the waveform within the dead time cannot be observed. This is why many people still insist that digital oscilloscopes are not as good as analog oscilloscopes.

　　The advantages of analog oscilloscopes are self-explanatory, including good real-time performance, simple principle, and low price. However, the instrument principle itself also contains flaws that will eventually be abandoned by the times. There are roughly the following:

　　1. Limited bandwidth: This is absolutely a fatal flaw. As mentioned earlier, the input signal of the analog oscilloscope is amplified to directly control the deflection of the electron gun of the CRT display. Although the bandwidth of the amplifier can be higher and higher, the deflection speed of the CRT electron gun is limited. For high-frequency signals, the speed of the electron gun cannot keep up with the signal changes. Therefore, it is really difficult to increase the bandwidth of the current analog oscilloscope. 2. Unable to store and analyze: Many old engineers know very well that to save waveforms with an analog oscilloscope, you need to take a photo with a camera. If you want to measure amplitude, period, and rise time, you can only do it manually. If you want to measure phase difference, power, etc., for digital oscilloscopes, you can only do it by checking a box, but for analog oscilloscopes, it is simply a physical job. 3. The triggering ability is too weak: basically only edge triggering is possible. Want pulse width triggering? Slope triggering? It's impossible! Don't open a graph to do template triggering, which is a brain-opening triggering method. 4. Unstable performance: After all, there are a lot of analog devices. After a long time, the indicators will be unstable, and the temperature drift will be much more serious than that of digital oscilloscopes. Digital oscilloscopes have gradually come to the fore since the 1980s. Especially with the development of high-speed ADC chips and digital processing technology, digital oscilloscopes have surpassed analog oscilloscopes in terms of bandwidth, triggering, analysis, and display. Now, almost all oscilloscopes on the market are digital oscilloscopes.

　　One point to emphasize here is still the dead time, which depends on the processing and display speed behind the digital oscilloscope. Although real-time processing is still not possible under the current technical level, the faster the processing speed, the fewer waveforms are lost. The performance indicator in this regard is called the waveform refresh rate. For a 200MHz bandwidth oscilloscope, almost all brands will be equipped with a 1G sampling rate, but the waveform refresh rate is one of the more important parameters. The higher the waveform refresh rate, the smaller the dead time of waveform observation.

　　In any case, it is an inevitable trend for digital oscilloscopes to replace analog oscilloscopes. In the stage of rapid development of electronic technology, I believe that the price advantage of analog oscilloscopes will gradually disappear.