The display circuit includes oscilloscope tube and its control circuit. Oscilloscope is a special kind of electron tube, which is an important part of oscilloscope. The oscilloscope tube consists of an electron gun, a deflection system and a fluorescent screen.
(1) electron gun
Electron guns are used to generate and form high-speed, bunched electron streams that bombard fluorescent screens to make them glow. It mainly consists of filament F, cathode K, control electrode G, first anode A1 and second anode A2. Except for filament, the structure of other electrodes is metal cylinder, and their axes are kept on the same axis. When the cathode is heated, electrons can be emitted along the axis; the control pole is a negative potential relative to the cathode, and 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 fluorescent screen. In order to improve the brightness of the 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 fluorescent screen in the modern oscilloscope.
The first anode adds a positive voltage of about several hundred volts to the cathode. A positive voltage higher than that of the first anode is added to the second anode. The electron beam passing through the control hole accelerates under the action of the high potential of the first and second anodes and moves at high speed towards the fluorescent screen. Because of the repulsion of charge, the electron beam will gradually disperse. By focusing the electric field between the first anode and the second anode, the electrons are re-assembled and converged at one point. If the potential difference between the first and second anodes is properly controlled, the focus will fall on the screen and a bright and fine dot will appear. Changing the potential difference between the first and second anodes can adjust the focusing of light points, which is the principle of the oscilloscope's "focusing" and "assistant focusing". The third anode is formed by coating a layer of graphite inside the cone of oscilloscope tube, usually with a high voltage. It has three functions: (1) to further accelerate the electrons passing through the deflection system, so that the electrons have enough energy to bombard the screen to obtain sufficient brightness; (2) to coat the graphite layer on the whole cone, it can play a role. Shielding effect; 3) Electron beam bombardment on 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. It consists of two pairs of parallel metal plates which are perpendicular to each other. They are called horizontal deflection plate and vertical deflection plate respectively. The movement of the electron beam in the horizontal and vertical directions is controlled respectively. When the electrons move between the deflection plates, if there is no voltage on the deflection plates and 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 at 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 emitted to the position of the fluorescent screen under the effect of the deflection electric field.
If the two deflecting plates are parallel to each other and their potential difference equals zero, the electron beam with velocity_in the deflecting plate space will move along the original direction (set as the axis direction) and hit the coordinate origin of the screen. If there is a constant potential difference between the two deflecting plates, an electric field will be formed between the deflecting plates. The electric field is perpendicular to the direction of the electrons'movement, and the electrons will deflect towards the deflecting plates with higher potential. In this way, in the space between the two deflecting plates, the electrons move tangentially along the parabola at this point. Finally, the electron lands at point A on the screen, which is a distance from the origin (0) of the screen. This distance is called deflection, expressed in y. The deflection y is proportional to the voltage Vy applied on the deflection board. Similarly, when a DC voltage is added to the horizontal deflection board, a similar situation occurs, except that the light point is deflected horizontally.
(3) Physical picture of fluorescent screen oscilloscope
The fluorescent screen is located at the end of the oscilloscope tube. Its function is to display the deflected electron beam for observation. The inner wall of the oscilloscope's fluorescent screen is coated with a layer of luminous substance, so the fluorescence appears on the fluorescent screen where it is impacted by high-speed electrons. The brightness of the spot depends on the number, density and velocity of the electron beam. When the voltage of the control electrode is changed, the number of electrons in the electron beam will change, and the brightness of the spot will also change. When using an oscilloscope, it is not advisable to fix a bright spot on the oscilloscope fluorescent screen. Otherwise, the fluorescent material at that spot will be burnt down by long-term electron impact, thus losing its luminous ability.