Oscilloscope Display Circuit

The display circuit includes two parts: an oscilloscope tube and a control circuit. Oscilloscope tube is a special type of electronic tube, which is an important component of oscilloscope. The oscilloscope tube consists of three parts: an electron gun, a deflection system, and a fluorescent screen.
(1) Electron gun
The electron gun is used to generate and form a high-speed, bunched electron stream that bombards the fluorescent screen to make it glow. It is mainly composed of a filament F, a cathode K, a control electrode G, a first anode A1, and a second anode A2. Except for the filament, the structure of the other electrodes is a metal cylinder, and their axis centers are maintained on the same axis. After being heated, the cathode 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 control aperture, which is to control the brightness of the light spots on the fluorescent screen. In order to improve the brightness of the light spot on the screen without reducing the sensitivity to electron beam deflection, a post acceleration electrode A3 is also added between the deflection system and the fluorescent screen in modern oscilloscope tubes.
The first anode applies a positive voltage of about several hundred volts to the cathode. A positive voltage higher than the first anode is applied to the second anode. The electron beam passing through the control hole is accelerated by the high potential of the first anode and the second anode, and moves at a high speed towards the fluorescent screen. Due to the repulsion of the same charge, the electron beam will gradually disperse. By focusing the electric field between the first anode and the second anode, electrons are reunited and converge at a point. Properly controlling the magnitude of the potential difference between the first anode and the second anode can make the focus just fall on the fluorescent screen, displaying a bright and small dot. Changing the potential difference between the first anode and the second anode can adjust the focus of the light spot, which is the principle of "focusing" and "auxiliary focusing" adjustment of the oscilloscope. The third anode is formed by coating a layer of graphite inside the cone of the oscilloscope tube, usually with a high voltage. It has three functions: ① to further accelerate the electrons passing through the deflection system, allowing the electrons to have sufficient energy to bombard the fluorescent screen to obtain sufficient brightness; ② Graphite layer coated on the entire cone can play a shielding role; ③ Electron beam bombardment of the fluorescent screen will generate secondary electrons, which can be absorbed by A3 at a high potential.