The basic structure of an oscilloscope

Basic structure of an oscilloscope
The main parts of an oscilloscope include an oscilloscope tube, a Y-axis amplifier with attenuator, an X-axis amplifier with attenuator, a scanning generator (sawtooth generator), a trigger synchronization and a power supply, etc. Its structural block diagram is shown in Figure 16-1. In order to meet the requirements of various measurements, the circuit composition of an oscilloscope is diverse and complex. Here we only introduce the main parts.
1. Oscilloscope tube
As shown in the figure, the oscilloscope tube mainly includes three parts: an electron gun, a deflection system and a fluorescent screen. All of them are sealed in a glass shell and evacuated to a high vacuum. The functions of each part are described below.
(1) Fluorescent screen: It is the display part of the oscilloscope. When the accelerated and focused electrons hit the fluorescent material, the fluorescent material coated on the screen will emit light, thereby showing the position of the electron beam. When the electrons stop acting, the fluorescent material will stop emitting light after a certain period of time, which is called the afterglow effect.

(2) Electron gun: It consists of five parts: filament H, cathode K, control grid G, first anode A1, and second anode A2. When the filament is energized, it heats the cathode. The cathode is a metal cylinder coated with oxide on the surface, which emits electrons when heated. The control grid is a cylinder with a small hole on the top, which is placed outside the cathode. Its potential is lower than that of the cathode, and it controls the electrons emitted by the cathode. Only electrons with a large initial velocity can pass through the small hole on the top of the grid and then rush to the fluorescent screen under the acceleration of the anode. The "brightness" adjustment on the oscilloscope panel is to adjust the potential to control the density of the electron flow directed to the fluorescent screen, thereby changing the brightness of the light spot on the screen. The anode potential is much higher than the cathode potential, and the electrons are accelerated by the electric field between them to form rays. When the potential between the control grid, the first anode, and the second anode is adjusted appropriately, the electric field in the electron gun has a focusing effect on the electron rays, so the first anode is also called the focusing anode. The second anode has a higher potential and is also called the accelerating anode. The "focusing" adjustment on the panel is to adjust the potential of the first anode so that the light spot on the fluorescent screen becomes a bright and clear small dot. Some oscilloscopes also have "auxiliary focusing", which is actually adjusting the potential of the second anode.
(3) Deflection system: It consists of two pairs of perpendicular deflection plates, one pair of vertical deflection plates Y and one pair of horizontal deflection plates X. When an appropriate voltage is applied to the deflection plates, the direction of the electron beam's movement is deflected when it passes through, thereby changing the position of the electron beam's light spot on the fluorescent screen.
It is easy to prove that the distance the light spot deflects on the fluorescent screen is proportional to the voltage applied to the deflection plates. Therefore, the voltage measurement can be converted into the measurement of the distance the light spot deflects on the screen. This is the principle of measuring voltage with an oscilloscope.
2. Signal amplifier and attenuator
The oscilloscope itself is equivalent to a multi-range voltmeter, and this function is achieved by a signal amplifier and attenuator. Since the sensitivity of the X and Y axis deflection plates of the oscilloscope itself is not high (about 0.1-1mm/V), when the signal applied to the deflection plates is too small, the small signal voltage must be amplified in advance before being applied to the deflection plates. For this purpose, an X-axis and Y-axis voltage amplifier is set. The function of the attenuator is to reduce the excessive input signal voltage to meet the requirements of the amplifier, otherwise the amplifier cannot work properly, the input signal is distorted, and even the instrument is damaged. For general oscilloscopes, attenuators are set on both the X-axis and the Y-axis to meet the needs of various measurements.
3. Scanning system
The scanning system is also called the time base circuit, which is used to generate a scanning voltage that changes linearly with time. The relationship between this scanning voltage and time is like a sawtooth, so it is called a sawtooth voltage. This voltage is amplified by the X-axis amplifier and added to the horizontal deflection plate of the oscilloscope tube to make the electron beam produce horizontal scanning. In this way, the horizontal coordinate on the screen becomes the time coordinate, and the waveform of the measured signal input on the Y-axis can be unfolded on the time axis. The scanning system is an important component necessary for the oscilloscope to display the measured voltage waveform.