Oscilloscope-based signal generator and uses of broadband radar signals
How an oscilloscope works
An oscilloscope is an electronic measuring instrument that uses the characteristics of electronic oscilloscope tubes to convert alternating electrical signals that cannot be directly observed by the human eye into images and display them on a fluorescent screen for measurement. It is an indispensable and important instrument for observing digital circuit experimental phenomena, analyzing problems in experiments, and measuring experimental results. The oscilloscope consists of an oscilloscope tube and power supply system, synchronization system, X-axis deflection system, Y-axis deflection system, delay scanning system, and standard signal source.
1. Oscilloscope tube
Cathode ray tube (CRT), referred to as oscilloscope tube, is the core of the oscilloscope. It converts electrical signals into light signals. As shown in Figure 1, the electron gun, deflection system and phosphor screen are sealed in a vacuum glass shell to form a complete oscilloscope tube.
(1) Fluorescent screen
Today's oscilloscope tube screens are usually rectangular planes, with a layer of phosphorescent material deposited on the inner surface to form a fluorescent film. A layer of evaporated aluminum film is often added to the fluorescent film. High-speed electrons pass through the aluminum film and hit the phosphor to form bright spots. The aluminum film has internal reflection, which is beneficial to improving the brightness of the bright spots. The aluminum film also has other functions such as heat dissipation.
When the electron bombardment stops, the bright spot cannot disappear immediately but must remain for a period of time. The time it takes for the brightness of a bright spot to drop to 10% of its original value is called the "afterglow time". Afterglow time shorter than 10μs is called very short afterglow, 10μs-1ms is short afterglow, 1ms-0.1s is medium afterglow, 0.1s-1s is long afterglow, and more than 1s is extremely long afterglow. Generally, oscilloscopes are equipped with medium persistence oscilloscope tubes, high-frequency oscilloscopes use short persistence, and low-frequency oscilloscopes use long persistence.
(2) Electron gun and focus
The electron gun consists of filament (F), cathode (K), grid (G1), front accelerating electrode (G2) (or second grid), first anode (A1) and second anode (A2). Its function is to emit electrons and form a very thin, high-speed electron beam. The filament is energized to heat the cathode, and the cathode emits electrons when heated.
The grid is a metal cylinder with a small hole on the top, which is placed outside the cathode. Since the gate potential is lower than the cathode, it controls the electrons emitted by the cathode. Generally, only a small number of electrons with a large initial velocity of movement can pass through the gate holes and rush to the fluorescent screen under the action of the anode voltage. Electrons with small initial velocity still return to the cathode.
If the gate potential is too low, all electrons return to the cathode, that is, the tube is turned off. Adjusting the W1 potentiometer in the circuit can change the gate potential and control the density of electron flow to the fluorescent screen, thereby adjusting the brightness of the bright spot. The first anode, the second anode and the front accelerating electrode are three metal cylinders on the same axis as the cathode. The front accelerating pole G2 is connected to A2, and the applied potential is higher than A1. The positive potential of G2 accelerates the electrons from the cathode towards the fluorescent screen.
As the electron beam travels from the cathode to the phosphor screen, it undergoes two focusing processes. The first focusing is completed by K, G1, and G2. K, K, G1, and G2 are called the first electronic lenses of the oscilloscope tube. The second focusing occurs in the G2, A1, and A2 areas. Adjusting the potential of the second anode A2 can make the electron beam converge at a point on the fluorescent screen. This is the second focusing. The voltage on A1 is called the focusing voltage, and A1 is also called the focusing pole. Sometimes adjusting the voltage of A1 still cannot achieve good focusing, and the voltage of the second anode A2 needs to be fine-tuned. A2 is also called the auxiliary focusing electrode.
(3) Deflection system
The deflection system controls the direction of the electron beam so that the light spot on the fluorescent screen changes with the external signal to depict the waveform of the measured signal. In Figure 8.1, two pairs of mutually perpendicular deflection plates Y1, Y2 and Xl, X2 form a deflection system. The Y-axis deflection plate is in the front and the X-axis deflection plate is in the back, so the Y-axis sensitivity is high (the measured signal is added to the Y-axis after processing). Voltage is applied to the two pairs of deflection plates respectively, so that an electric field is formed between the two pairs of deflection plates, which controls the deflection of the electron beam in the vertical and horizontal directions respectively.
