What is an oscilloscope? How to use an oscilloscope

Oscilloscope, as its name suggests, is a machine that displays waveforms. It is also called "the eyes of electronic engineers". Its basic function is to display the actual waveform of the measured signal on the screen for engineers to find and locate problems or evaluate system performance, etc. Its development has also gone through two eras: analog and digital. Let's take a look at the picture first, as shown in Figure 1.

Figure 1 imitates an oscilloscope, a digital oscilloscope, and an oscilloscope probe

Nowadays, analog oscilloscopes have basically been chosen, and now it is the world of digital oscilloscopes. Similarly, I will only use digital oscilloscopes as examples to explain.

A digital oscilloscope is more accurately called a digital storage oscilloscope, or DSO (Digital Storage Oscilloscope). The "storage" here does not mean that it can store waveforms on media such as USB flash drives, but refers to the instant display characteristics of an analog oscilloscope. An analog oscilloscope relies on a cathode ray tube (CRT, commonly known as an electron gun) to emit an electron beam, which deflects under the magnetic field formed by the measured signal and then reflects the waveform of the measured signal on the screen. This process is instantaneous and there is basically no storage process. The principle of a digital oscilloscope is this: first, the oscilloscope uses the front-end ADC to actively sample the measured signal, and the sampling speed can generally reach hundreds of M to several G times per second, which is quite fast; and the back-end display component of the oscilloscope is an LCD screen, and the update rate of the LCD screen is generally only tens to more than a hundred Hz; thus, the data sampled by the front end cannot be reflected on the screen in real time, so the storage link is born: the oscilloscope temporarily stores the data sampled by the front end in the internal memory, and reads the data from the memory when the display is updated, and uses this level of storage to deal with the speed difference between the front-end sampling and the back-end display.

When many people first see an oscilloscope, they may be intimidated by the many buttons on the panel. In addition, oscilloscopes are generally expensive, so they are afraid of using them. This is unnecessary, because although the oscilloscope looks complicated, in practice, it is not complicated to use its basic function - displaying waveforms. It only takes three or four steps to get it done. Nowadays, the complexity of oscilloscopes is caused by the addition of many auxiliary functions. These auxiliary functions naturally have their value. Using them skillfully and flexibly can achieve twice the result with half the effort. As beginners, let's not talk about these first. We only use its most basic and fundamental functions.

Oscilloscope application diagram

Similar to a multimeter, to use an oscilloscope, you must first connect it to the system under test, using an oscilloscope probe, as shown in Figure 20-4. An oscilloscope generally has 2 or 4 channels (usually marked with numbers 1 to 4, and the remaining probe socket is externally triggered and generally not used), their low bits are equal, and you can choose them at will, plug the probe into one of the channels, and the small clip at the other end of the probe is connected to the reference ground of the system under test (here you must pay attention to a problem: the clip on the oscilloscope probe is directly connected to the earth, that is, the ground wire on the three-pin plug, so if there is a voltage difference between the reference ground of the system under test and the earth, it will cause damage to the oscilloscope or the system under test), the probe touches the point under test, so the oscilloscope can collect the voltage waveform at that point (ordinary probes cannot be used to measure current, and special current probes must be selected to measure current).

Next, you need to adjust the buttons on the oscilloscope panel to make the measured waveform appear on the screen at an appropriate size. You only need to adjust the oscilloscope parameters according to the two major elements of a signal - amplitude and period (frequency and period are conceptually equivalent), as shown in Figure 2.

Figure 2 Oscilloscope amplitude and time axis knobs

As shown in the figure above, the knob above each channel socket is used to adjust the amplitude of the channel, that is, the adjustment of the vertical size of the waveform. By rolling them, the voltage value represented by each vertical grid on the oscilloscope screen can be changed, so it can be called "volt grid" adjustment, as shown in the following two comparison pictures: the left picture is 1V/grid, and the right picture is 500mV/grid. The amplitude of the waveform in the left picture occupies 2.5 grids, so 2.5V, and the amplitude of the waveform in the right picture occupies 5 grids, also 2.5V. It is recommended to adjust the waveform to the position in the right picture, because now the waveform occupies a larger space of the entire measurement plan, which can improve the accuracy of waveform measurement, as shown in Figure 3.

Figure 3 Oscilloscope voltage characteristic comparison diagram

In addition to the volts knob on the top of Figure 3, you will usually find a knob of the same size on the panel (not necessarily in the position shown in Figure 20-6). This knob is used to adjust the cycle, that is, the horizontal size of the waveform. By rolling it, you can change the time value represented by each horizontal grid on the oscilloscope screen, so it can be called the "second grid" adjustment, as shown in the following two comparison pictures: the left picture is 500us/grid, and the right picture is 200us/grid. In the left picture, one cycle occupies 2 grids, and the cycle is 1ms, that is, the frequency is 1KHz. In the right picture, one cycle occupies 5 grids, which is also 1ms, that is, 1KHz. There is no more reasonable question here. Treat the specific questions in detail, and they are all very reasonable, as shown in Figure 4.

Figure 4 Oscilloscope second characteristic comparison chart

In many cases, if only the above two adjustments are made, a waveform can be seen, but this waveform is very unstable, with chaotic distribution and overlap, making it difficult to see clearly, as shown in Figure 5.

Figure 5: Improper adjustment of the oscilloscope trigger level

This is because the trigger of the oscilloscope is not adjusted properly, so what is trigger? To put it simply, the so-called trigger is to set a benchmark so that the collection and display of the waveform revolve around this benchmark. The most commonly used trigger setting is based on the level (it can also be based on other quantities such as time, the principle is the same). Let's look at the waveforms above. There is always a T and a small arrow on the left. T means trigger. The voltage value corresponding to the position pointed by this small arrow is the trigger level at that time. The oscilloscope always stores the previous and next parts when the waveform passes through this level and finally displays them, so you can see the waveforms shown in Figures 4 and 5. As shown in Figure 6, we can see that no matter what, the waveform will not pass through the position indicated by T, that is, it cannot reach the trigger level for a long time, so the waveform without the benchmark looks unstable. How to adjust the position of this trigger level? Find a knob marked with Trigger on the oscilloscope panel, as shown in the figure below. Turning this knob can change the position of this T.

Figure 6 Oscilloscope trigger knob

In addition to changing the trigger level value, you can also set the trigger method: for example, choose rising edge or falling edge trigger, that is, choose to let the waveform pass the trigger level when it increases upward or pass the trigger level when it decreases downward to complete the trigger. These settings are generally completed through the buttons in the Trigger column and the convenient menu keys on the screen.

With just the above three or four steps, you can use the basic functions of the oscilloscope and use it to check the various signals of the microcontroller system. For example, if the system does not work after power-on, use it to test whether the waveform of the crystal oscillator pin is normal. It should be noted that the waveform on the crystal oscillator pin is not a square wave, but more like a sine wave, and the waveforms on the two pins of the crystal oscillator are different. The one with a smaller amplitude is used as input, and the one with a larger amplitude is used as output, as shown in Figure 7.

Figure 7 Crystal oscillator waveform measured by oscilloscope