The oscilloscope consists of 3 parts: the electron gun, the deflection system and the fluorescent screen.
(1) Electron gun
The electron gun is used to generate and form a high-speed, poly-beam electron flow, to bombard the fluorescent screen to make it emit light. It is mainly composed of filament F, cathode K, control pole G, the first anode A1, the second anode A2. In addition to the filament, the structure of the remaining electrodes are metal cylinders, and their axes are kept on the same axis. After the cathode is heated, it can emit electrons along the axis; the control pole is negative potential relative to the cathode, and changing the potential can change the number of electrons passing through the small holes of the control pole, that is, to control the brightness of the light spot on the fluorescent screen. In order to improve the brightness of the point of light on the screen, without reducing the sensitivity of the electron beam deflection, modern oscilloscope, between the deflection system and the fluorescent screen is also added a rear accelerating electrode A3.
A positive voltage of about several hundred volts is applied to the first anode with respect to the cathode. A higher positive voltage than that of the first anode is applied to the second anode. The electron beam passing through the small hole in the control pole is accelerated under the action of the high potentials of the first and second anodes and moves at high speed in the direction of the fluorescent screen. Due to the charge of the same sex repulsion, the electron beam will gradually spread 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. Appropriate control of the size of the potential difference between the first anode and the second anode, you can make the focus just fall on the fluorescent screen, showing a bright small dot. Change the potential difference between the first anode and the second anode, can play a role in regulating the focus of the point of light, which is the oscilloscope's "focus" and "auxiliary focus" principle of adjustment. The third anode is the oscilloscope cone coated with a layer of graphite formed, usually with a high voltage, it has three roles: ① through the deflection system after the electrons are further accelerated, so that the electrons have enough energy to bombard the screen in order to obtain sufficient brightness; ② graphite layer coated in the cone, can play a shielding role; ③ bombardment of the screen by the electron beam will produce secondary electrons, at a high potential A3 can be absorbed these electrons. absorb these electrons.
(2) deflection system
Oscilloscope deflection system is mostly electrostatic deflection, which consists of two pairs of mutually perpendicular parallel metal plate, respectively, known as the horizontal deflection plate and vertical deflection plate. Respectively control the movement of the electron beam in the horizontal and vertical directions. When electrons move between the deflector plates, if there is no voltage applied to the deflector plates and no electric field between the plates, the electrons that enter the deflection system after leaving the second anode will move in the axial direction and shoot to the centre of the screen. If there is a voltage on the deflector plate, there is an electric field between the deflector plates, and the electrons entering the deflection system will be shot to the specified position of the fluorescent screen under the action of the deflecting electric field.
If the two deflector plates are parallel to each other and their potential difference is equal to zero, then the electron beam passing through the space of the deflector plates with velocity υ will move in the original direction (set to the direction of the axis) and hit the origin of the coordinates of the fluorescent screen. If there is a constant potential difference between the two deflector plates, then the deflector plate between the formation of an electric field, the electric field and the direction of motion of the electrons perpendicular to the direction of motion, so the electrons will be deflected towards the deflector plate with a higher potential. Thus, in the space between the two deflector plates, the electrons move tangentially along the parabola at this point. Finally, the electron lands at point A on the phosphor screen, which is some distance from the origin of the screen (0), and this distance is called the deflection, denoted by y. The deflection y is proportional to the voltage Vy applied to the deflector plate. Similarly, when a DC voltage is added to the horizontal deflection plate, a similar situation occurs, except that the point of light is deflected in the horizontal direction.
(3) Fluorescent Screen
The fluorescent screen is located at the end of the oscilloscope, and its function is to display the deflected electron beam for observation. The inside wall of the oscilloscope's fluorescent screen is coated with a layer of luminescent material, so that the locations on the screen that are impacted by high-speed electrons show fluorescence. The brightness of the spot is determined by the number and density of the electron beam and its speed. Change the voltage of the control pole, the number of electrons in the electron beam will be changed, the brightness of the light spot will also change. In the use of oscilloscopes, it is not advisable to let a very bright point of light fixed in the oscilloscope fluorescent screen in a position, otherwise the point of the fluorescent material will be burned due to the long-term impact of electrons, thus losing the ability to emit light.
Coated with different fluorescent substances of the fluorescent screen, the impact of electrons will show a different colour and different afterglow time, usually for the observation of general signal waveforms with a green light, is the afterglow oscilloscope, for the observation of non-periodic and low-frequency signals with an orange-yellow light, is a long afterglow oscilloscope; for the photographic oscilloscope, generally used in the short afterglow oscilloscope with a blue hair.
