About the internal structure and power supply test of the oscilloscope tube

In digital circuit experiments, several instruments and meters need to be used to observe experimental phenomena and results. Commonly used electronic measuring instruments include multimeters, logic pens, ordinary oscilloscopes, storage oscilloscopes, logic analyzers, etc. Multimeters and logic pens are relatively simple to use, while logic analyzers and storage oscilloscopes are not yet very commonly used in digital circuit teaching experiments. The oscilloscope is a very widely used and relatively complex instrument. This chapter introduces the principles and usage of the oscilloscope from the perspective of use.

1 Working principle of oscilloscope

An oscilloscope is an electronic measuring instrument that uses the characteristics of an electronic oscilloscope 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, delayed 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.

Figure 1 Internal structure and power supply diagram of oscilloscope tube

2). 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 the bright spot to drop to 10% of its original value is called "afterglow time". The afterglow time is shorter than 10μs, which is 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.

Due to the different phosphorescent materials used, different colors of light can be emitted from the fluorescent screen. Generally, oscilloscopes mostly use green-emitting oscilloscope tubes to protect people's eyes.

3). 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 electron flow density directed 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 to 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 on 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.

4). 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.

5). Oscilloscope power supply

In order for the oscilloscope tube to work properly, there are certain requirements for the power supply. It is stipulated that the potential between the second anode and the deflection plate is similar, and the average potential of the deflection plate is zero or close to zero. The cathode must operate at a negative potential. The grid G1 has a negative potential (-30V~-100V) relative to the cathode, and is adjustable to achieve brightness adjustment. The first anode has a positive potential (about +100V~+600V) and should also be adjustable for focus adjustment. The second anode is connected to the front accelerating electrode, and the opposite cathode is at a positive high voltage (about +1000V). The adjustable range relative to the ground potential is ±50V. Since the current of each electrode of the oscilloscope tube is very small, it can be powered by a public high voltage through a resistor divider.

The measured signal ① is connected to the "Y" input terminal, and is properly attenuated by the Y-axis attenuator before being sent to the Y1 amplifier (preamplifier), and push-pull output signals ② and ③. After the delay stage, it is delayed by Г1 time and reaches the Y2 amplifier. After amplification, sufficiently large signals ④ and ⑤ are generated and added to the Y-axis deflection plate of the oscilloscope tube. In order to display a complete stable waveform on the screen, introduce the Y-axis measured signal ③ into the trigger circuit of the X-axis system, generate a trigger pulse ⑥ at a certain level value of the positive (or negative) polarity of the introduced signal, and start Sawtooth wave scanning circuit (time base generator) generates scanning voltage ⑦. Since there is a time delay Г2 from triggering to starting scanning, in order to ensure that the X-axis starts scanning before the Y-axis signal reaches the fluorescent screen, the delay time Г1 of the Y-axis should be slightly larger than the delay time Г2 of the X-axis. The scanning voltage ⑦ is amplified by the X-axis amplifier to generate push-pull outputs ⑨ and ⑩, which are added to the X-axis deflection plate of the oscilloscope tube. The z-axis system is used to amplify the positive path of the scanning voltage and turn it into a forward rectangular wave, which is sent to the oscilloscope grid. This allows the waveform displayed during the scan to have a certain fixed brightness, and is wiped out during the return scan.

The above is the basic working principle of an oscilloscope. Dual-trace display uses electronic switches to display two different measured signals input from the Y-axis on the fluorescent screen respectively. Due to the persistence of vision of the human eye, when the conversion frequency reaches a certain level, you will see two stable and clear signal waveforms.

There is often an accurate and stable square wave signal generator in the oscilloscope for calibration of the oscilloscope.

2. Use of oscilloscope

This section introduces how to use the oscilloscope. There are many types and models of oscilloscopes with different functions. In digital circuit experiments, 20MHz or 40MHz dual-trace oscilloscopes are commonly used. These oscilloscopes are used in much the same way. This section does not target a certain model of oscilloscope, but only conceptually introduces the common functions of oscilloscopes in digital circuit experiments.

1) Fluorescent screen

The fluorescent screen is the display part of the oscilloscope tube. There are multiple scale lines in the horizontal and vertical directions on the screen, indicating the relationship between the voltage and time of the signal waveform. The horizontal direction indicates time, and the vertical direction indicates voltage. The horizontal direction is divided into 10 grids, the vertical direction is divided into 8 grids, and each grid is divided into 5 parts. The vertical direction is marked with 0%, 10%, 90%, 100% and other marks, and the horizontal direction is marked with 10% and 90% marks, which are used to measure parameters such as DC level, AC signal amplitude, delay time, etc. The voltage value and time value can be obtained based on the number of grids occupied by the measured signal on the screen multiplied by the appropriate proportional constant (V/DIV, TIME/DIV).