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Operating Principle of Oscilloscope
I. Operating principle of oscilloscope:
The oscilloscope uses narrow electron beams composed of high-speed electrons to strike on the screen coated with fluorescent substances to produce tiny light spots. Under the action of the measured signal, the electron beam acts as the tip of a pen, which can depict the change curve of the instantaneous value of the measured signal on the screen. The oscilloscope can observe different kinds of signals. The waveform curve of signal amplitude changing with time can also be used to test various electric quantities, such as voltage, current, frequency, phase difference, amplitude modulation and so on.
(1) Oscilloscope consists of five basic components: display circuit, vertical (Y-axis) amplifier circuit, horizontal (X-axis) amplifier circuit, scanning and synchronization circuit, power supply circuit. The principle and function block diagram of ordinary oscilloscope is shown in Fig. 5-1.
1. display circuit
The display circuit consists of oscilloscope tube and its control circuit. Oscilloscope is a special electronic tube and an important part of oscilloscope. The basic principle diagram of oscilloscope tube is shown in Fig. 5-2. It can be seen from the diagram that the oscilloscope tube consists of three parts: electron gun, deflection system and fluorescent screen.
(1) electron gun
Electron gun is used to generate and form high-speed, bunched electron flow to bombard the fluorescent screen to make it luminous. It mainly consists of filament F, cathode K, control electrode G, first anode A1, second anode A2. Except filament, the structure of the other electrodes are metal cylinders, and their axes are kept on the same axis. When the cathode is heated, it can be along the axis. To emit electrons; the control pole is a negative potential relative to the cathode. Changing the potential can change the number of electrons through controlling the very small hole, that is, to control the brightness of the spot on the screen. In order to improve the brightness of the spot on the screen and not to reduce the sensitivity to the deflection of the electron beam, modern oscilloscope tubes are added between the deflection system and the screen. A rear acceleration electrode A3.
The first anode has a positive voltage of about several hundred volts for the cathode. The second anode has a higher positive voltage than the first anode. The electron beam passing through the control hole accelerates under the action of the high potential of the first anode and the second anode, and moves at a high speed towards the fluorescent screen. Because of the repellency of the charges, the electron beams will move one by one. By focusing the electric field between the first anode and the second anode, electrons gather again and converge at one point. Proper control of the potential difference between the first anode and the second anode will enable the focus to fall on the fluorescent screen, showing a bright and tiny dot. Changing the distance between the first anode and the second anode. The third anode is formed by coating graphite inside the cone of the oscilloscope. It usually has a high voltage. It has three functions: 1. Accelerating the electrons passing through the deflection system further and making the electrons sufficient. Enough energy to bombard the screen to get enough brightness; 2) graphite coating on the whole cone can play a shielding role; 3) electron beam bombardment of the screen will produce secondary electrons, which can be absorbed by A 3 at high potential.
(2) deflection system
The deflection system of oscilloscope tube is mostly electrostatic deflection type. It consists of two pairs of vertical parallel metal plates, which are called horizontal deflection plate and vertical deflection plate respectively. The movement of electron beam in horizontal and vertical directions is controlled respectively. When electrons move between deflection plates, if there is no voltage on the deflection plate, there is no electricity between deflection plates. Fields, electrons entering the deflection system after leaving the second anode will move along the axis and shoot at the center of the screen. If there is a voltage on the deflection board, there is an electric field between the deflection plates. Electrons entering the deflection system will shoot at the position of the fluorescent screen under the effect of the deflection electric field.
(3) fluorescent screen
The fluorescent screen is located at the end of the oscilloscope. Its function is to display deflected electron beams for observation. There is a layer of luminous substance coated on the inner wall of the oscilloscope's fluorescent screen, so the fluorescence appears at the place where the fluorescent screen is impacted by high-speed electrons. At this time, the brightness of the spot depends on the number, density and speed of the electron beams. When the voltage of the control pole is changed, the number of electrons in the electron beam will change, and the brightness of the light spot will also change. When using the oscilloscope, it is not appropriate to fix the bright spot in a position of the oscilloscope fluorescent screen, otherwise the fluorescent material at the spot will be burned by long-term electron impact, thus losing its luminous ability.
Fluorescent screens coated with different fluorescent substances will show different colours and different afterglow time when they are impacted by electrons. Usually, green light is used to observe general signal waveforms. They are medium afterglow oscilloscopes, orange yellow light is used to observe non-periodic and low frequency signals, and long afterglow oscilloscopes are used for photography. In general, blue-emitting short afterglow oscilloscopes are used.