Oscilloscope display circuit composition
The display circuit includes two parts: the oscilloscope tube and its control circuit. The oscilloscope is a special type of electronic tube and is an important part of the oscilloscope. The oscilloscope tube consists of three parts: electron gun, deflection system and fluorescent screen.
(1) Electron gun
The electron gun is used to generate and form a high-speed, focused electron stream to bombard the fluorescent screen and cause it to emit light. It mainly consists of filament F, cathode K, control electrode G, first anode A1, and second anode A2. Except for the filament, the structures of the other electrodes are metal cylinders, and their axes are kept on the same axis. After the cathode is heated, it can emit electrons along the axial direction; the control electrode has a negative potential relative to the cathode. Changing the potential can change the number of electrons passing through the extremely small holes, which is to control the brightness of the light spots on the fluorescent screen. In order to increase the brightness of the light spot on the screen without reducing the sensitivity to electron beam deflection, a post-acceleration electrode A3 is added between the deflection system and the phosphor screen in modern oscilloscope tubes.
The first anode has a positive voltage of about several hundred volts applied to the cathode. A higher positive voltage than the first anode is applied to the second anode. The electron beam passing through the extremely small hole is accelerated by the high potential of the first anode and the second anode and moves toward the fluorescent screen at high speed. Because like charges repel each other, the electron beam gradually spreads out. Through the focusing effect of the electric field between the first anode and the second anode, the electrons are regrouped and converge at one point. By properly controlling the potential difference between the first anode and the second anode, the focus can just fall on the fluorescent screen and a bright and tiny dot will appear. Changing the potential difference between the first anode and the second anode can adjust the focus of the light spot. This is the principle of the "focus" and "auxiliary focus" adjustment of the oscilloscope. The third anode is formed by coating the inside of the oscilloscope cone with a layer of graphite. It is usually applied with a very high voltage. It has three functions: 1. It further accelerates the electrons after passing through the deflection system, so that the electrons have enough energy to Bombard the fluorescent screen to obtain sufficient brightness; ② The graphite layer is coated on the entire cone, which can play a shielding role; ③ The electron beam bombards the fluorescent screen to generate secondary electrons, and A3 at high potential can absorb these electrons.
(2) Deflection system
Most of the deflection systems of oscilloscope tubes are electrostatic deflection types, which consist of two pairs of parallel metal plates perpendicular to each other, called horizontal deflection plates and vertical deflection plates respectively. Control the movement of the electron beam in the horizontal and vertical directions respectively. When electrons move between the deflection plates, if no voltage is applied to the deflection plates and there is no electric field between the deflection plates, the electrons entering the deflection system after leaving the second anode will move along the axis and shoot towards the center of the screen. If there is a voltage on the deflection plate, there is an electric field between the deflection plates, and the electrons entering the deflection system will be directed to the designated position of the fluorescent screen under the action of the deflection electric field.
If the two deflection plates are parallel to each other and their potential difference is equal to zero, then the electron beam with speed υ passing through the deflection plate space will move along the original direction (set as the axis direction) and hit the coordinate origin of the fluorescent screen. . If there is a constant potential difference between the two deflection plates, an electric field will be formed between the deflection plates. This electric field is perpendicular to the direction of movement of the electrons, so the electrons will deflect towards the deflection plate with a higher potential. In this way, in the space between the two deflection plates, the electrons move tangentially along the parabola at this point. Finally, the electron lands at point A on the fluorescent screen. This point A is a certain distance from the origin (0) of the fluorescent screen. This distance is called the deflection amount, represented by y. The deflection amount y is proportional to the voltage Vy applied to the deflection plate. In the same way, when a DC voltage is applied to the horizontal deflection plate, a similar situation occurs, except that the light spot is deflected in the horizontal direction.
(3) Fluorescent screen
The fluorescent screen is located at the terminal of the oscilloscope tube. Its function is to display the deflected electron beam for observation. The inner wall of the oscilloscope's phosphor screen is coated with a layer of luminescent material, so the locations on the phosphor screen that are impacted by high-speed electrons emit fluorescence. The brightness of the light spot at this time depends on the number, density and speed of the electron beam. When the voltage of the control electrode is changed, the number of electrons in the electron beam will change accordingly, and the brightness of the light spot will also change. When using an oscilloscope, it is not advisable to allow a very bright light point to appear fixedly at one position on the oscilloscope tube's fluorescent screen, otherwise the fluorescent material at that point will be burned out due to long-term impact by electrons, thus losing its ability to emit light.
Fluorescent screens coated with different fluorescent substances will display different colors and different afterglow times when impacted by electrons. Usually, the one used to observe general signal waveforms emits green light and is a medium-afterglow oscilloscope tube for observing non-periodic For high-frequency and low-frequency signals, the oscilloscope tube that emits orange-yellow light and is a long-persistence oscilloscope is generally used. In oscilloscopes used for photography, short-permanence oscilloscope tubes that emit blue light are generally used.
