The most comprehensive introduction, use and operation of oscilloscopes

　　An oscilloscope, as its name suggests, is a machine that displays waveforms. It is also known as the "eyes of electronic engineers." Its core function is to display the actual waveform of the measured signal on the screen so that engineers can 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.

　　What is an oscilloscope used for?

　　1. Can measure the voltage amplitude of DC signal and AC signal

　　2. The period of the AC signal can be measured and used to convert the frequency of the AC signal.

　　3. Can display the waveform of AC signal.

　　4. Signal measurement can be performed using two channels separately.

　　5. The waveforms of two signals can be displayed on the screen at the same time, that is, the dual-trace measurement function. This function can measure the phase difference between the two signals and the difference in shape between the waveforms.

　　How to use an oscilloscope_Oscilloscope usage 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 extra probe socket is an external trigger, which is generally not used). Their low bits are equivalent, so you can choose any one of them. Plug the probe into one of the channels, and connect the small clip at the other end of the probe to the reference ground of the system under test (one thing must be noted here: 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 that 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).

　　The next step is to adjust the buttons on the oscilloscope panel so that the measured waveform is displayed 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, to adjust the size of the waveform in the vertical direction. By turning them, you can change the voltage value represented by each vertical grid on the oscilloscope screen, 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 it is 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 right picture, because at this time the waveform occupies a larger space in the entire measurement range, which can improve the accuracy of waveform measurement, as shown in Figure 3.

　　Figure 3 Oscilloscope Volt Adjustment Comparison

　　In addition to the volts knob usually above 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 turning 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. They are all very reasonable when dealing with specific issues, as shown in Figure 4.

　　Figure 4 Oscilloscope second-grid adjustment comparison chart

　　In many cases, if only the above two adjustments are made, a waveform can be seen, but this waveform is very unstable, shaking left and right, overlapping each other, making it unclear, as shown in Figure 5.

　　Figure 5 Schematic diagram of improper adjustment of oscilloscope trigger level

　　This is because the trigger of the oscilloscope is not adjusted properly. So what is a trigger? To put it simply, the so-called trigger is to set a benchmark so that the acquisition 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 reason 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 the small arrow is the current trigger level. When the waveform passes through this level, the oscilloscope always stores and displays the previous and subsequent parts, 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 the position indicated by T, that is, it will never reach the trigger level, 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 value of the trigger level, you can also set the trigger method: for example, choose rising edge or falling edge trigger, that is, choose whether to pass the trigger level when the waveform increases upward or 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.

　　As long as you go through the above three or four steps, you can apply the core functions of the oscilloscope and use it to observe various signals of the microcontroller system. For example, if the system does not run 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