Introduction to the principle of oscilloscope display waveform

　　Although oscilloscopes have many types or models, they all contain several basic parts in structure: oscilloscope tube, horizontal amplifier, vertical amplifier, sweep generator, trigger synchronization and DC power supply, etc.

　　The oscilloscope tube is a key component of the oscilloscope. When the electron gun is heated and emits an electron beam, it is accelerated by the electric field and hits the fluorescent screen to form a bright spot. Before reaching the fluorescent screen, the electron beam must pass through two pairs of mutually perpendicular electric deflection plates. If there is no deflection electric field, the electron beam will hit the center of the fluorescent screen; if a deflection electric field is applied, the position of the electron beam (bright spot) will shift.

　　If only an alternating sinusoidal voltage Uy=U0sinωt is applied to the vertical (Y-axis) deflection plate, the bright spot of the electron beam will move back and forth in the vertical direction as the voltage changes. Since Ux=0, the light spot has no displacement in the X-axis direction, and a vertical bright line will be displayed on the fluorescent screen. If only a sawtooth wave scanning voltage Ux=KT proportional to time is applied to the horizontal (X-axis) deflection plate (it can be generated by the scanning generator in the oscilloscope), the electron beam will periodically move from one side to the other at a uniform speed in the horizontal direction. If the period of the sawtooth wave is long, the movement of the electron beam can be seen on the fluorescent screen. If the period of the sawtooth wave is short enough, only a horizontal bright line will be displayed on the fluorescent screen.

　　If a sawtooth voltage is applied to the horizontal deflection plate and a periodically changing electric signal is applied to the vertical deflection plate, the electron beam will move horizontally at a uniform speed while also moving vertically with the change of the periodic electric signal. The fluorescent screen will display the changing pattern of the electric signal applied to the vertical deflection plate - the waveform. When the period of the vertical electric signal is the same as or an integer multiple of the period of the horizontal sawtooth voltage, the pattern on the fluorescent screen will be synchronously reproduced through repeated scanning, thereby displaying a stable waveform of the vertical electric signal. Point a″ on the fluorescent screen corresponds to point a of the voltage applied on the Y-axis and point a′ of the voltage applied on the X-axis, and so on. The light spots on the fluorescent screen just depict the changing pattern of the voltage on the Y-axis over time.

　　If the period of the voltage applied on the Y axis is not equal to or an integer multiple of the period of the scanning voltage, a stable graph cannot be formed, as shown in Figure 14-3. In order to obtain a stable signal waveform on the fluorescent screen. A general oscilloscope has a "step circuit" that uses the signal to be measured to control the frequency of the scanning generator, which is called trigger scanning. That is, the scanning starting point of the sawtooth scanning voltage automatically follows the measured signal and displays the measured signal. This is "internal step"; or a step voltage is connected from the outside and added to the sawtooth generator to force the sawtooth wave to maintain an integer multiple relationship with the frequency of the signal to be measured. This is called "external step".