A deep dive into the internal principles and structure of an oscilloscope

The oscilloscope is a widely used and relatively complex instrument. This chapter introduces the principles and usage of the oscilloscope from the perspective of usage.

01Oscilloscope
Working Principle

An oscilloscope is an electronic measuring instrument that uses the characteristics of an electronic oscilloscope tube to convert alternating electrical signals that cannot be directly observed by the human eye into images and displays them on a fluorescent screen for measurement. It is an important instrument that is essential for observing digital circuit experimental phenomena, analyzing experimental problems, and measuring experimental results. An 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

The cathode ray tube (CRT), also known as the oscilloscope, is the core of the oscilloscope. It converts electrical signals into optical signals. As shown in Figure 1, the electron gun, deflection system and fluorescent screen are sealed in a vacuum glass shell to form a complete oscilloscope.

Figure 1 Internal structure and power supply diagram of oscilloscope

(1) Fluorescent screen

The current oscilloscope screen is usually a rectangular plane, with a layer of phosphor material deposited on the inner surface to form a fluorescent film. A layer of evaporated aluminum film is often added on the fluorescent film. High-speed electrons pass through the aluminum film, hit the phosphor and emit light to form a bright spot. The aluminum film has an internal reflection effect, which is beneficial to increase the brightness of the bright spot. The aluminum film also has other functions such as heat dissipation.

When the electron bombardment stops, the bright spot cannot disappear immediately but remains for a while. The time it takes for the brightness of the bright spot to drop to 10% of the original value is called the "afterglow time". Afterglow time shorter than 10μs is very short afterglow, 10μs-1ms is short afterglow, 1ms-0.1s is medium afterglow, 0.1s-1s is long afterglow, and greater than 1s is very long afterglow. General oscilloscopes are equipped with medium afterglow oscilloscope tubes, high-frequency oscilloscopes use short afterglow, and low-frequency oscilloscopes use long afterglow.

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

(2) Electron gun and focusing

The electron gun consists of a filament (F), a cathode (K), a grid (G1), a front accelerating electrode (G2) (or a second grid), a first anode (A1) and a second anode (A2). Its function is to emit electrons and form a very thin high-speed electron beam. When the filament is energized, it heats the cathode, which then emits electrons.

The grid is a metal cylinder with a small hole on the top, which is placed outside the cathode. Since the grid potential is lower than that of the cathode, it controls the electrons emitted by the cathode. Generally, only a small number of electrons with a large initial velocity can pass through the grid hole under the action of the anode voltage and rush to the fluorescent screen. Electrons with a small initial velocity still return to the cathode.

If the grid potential is too low, all electrons return to the cathode, that is, the tube is cut off. By adjusting the W1 potentiometer in the circuit, the grid potential can be changed to control the density of the electron flow 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 electrode G2 is connected to A2, and the applied potential is higher than A1. The positive potential of G2 accelerates the cathode electrons to the fluorescent screen.

The electron beam goes through two focusing processes when it runs from the cathode to the fluorescent screen. The first focusing is completed by K, G1, and G2. K, K, G1, and G2 are called the first electron lens of the oscilloscope. The second focusing occurs in the G2, A1, and A2 area. 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 electrode. Sometimes adjusting the A1 voltage still cannot meet the requirements of good focusing, and it is necessary to fine-tune the voltage of the second anode A2, which 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 depicts the waveform of the measured signal as the external signal changes. In Figure 8.1, two pairs of mutually perpendicular deflection plates Y1, Y2 and X1, X2 form the deflection system. The Y-axis deflection plate is in 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). Voltages are applied to the two pairs of deflection plates respectively, so that electric fields are formed between the two pairs of deflection plates, respectively controlling the deflection of the electron beam in the vertical and horizontal directions.

(4) Oscilloscope power supply

In order for the oscilloscope 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 close, and the average potential of the deflection plate is zero or close to zero. The cathode must work at a negative potential. The grid G1 is negative relative to the cathode (-30V~-100V), and it is adjustable to achieve brightness adjustment. The first anode is positive (about +100V~+600V), which should also be adjustable for focus adjustment.

The second anode is connected to the front accelerating electrode, and the cathode is a positive high voltage (about +1000V), and the adjustable range relative to the ground potential is ±50V. Since the current of each electrode of the oscilloscope tube is very small, a common high voltage can be used to supply power through a resistor divider.

02Basic
components of an oscilloscope

As can be seen from the previous section, as long as the voltage on the X-axis deflection plate and the Y-axis deflection plate is controlled, the shape of the graph displayed by the oscilloscope can be controlled. We know that an electronic signal is a function of time f(t), which changes with time. Therefore, as long as a voltage proportional to the time variable is added to the X-axis deflection plate of the oscilloscope, and the measured signal (after proportional enlargement or reduction) is added to the y-axis, the graph of the measured signal changing with time will be displayed on the oscilloscope screen. Among electrical signals, the signal that is proportional to the time variable over a period of time is a sawtooth wave.

The basic block diagram of the oscilloscope is shown in Figure 2. It consists of five parts: oscilloscope tube, Y-axis system, X-axis system, Z-axis system and power supply.

Figure 2 Basic block diagram of an oscilloscope

The measured signal ① is connected to the "Y" input terminal, and after being properly attenuated by the Y-axis attenuator, it is sent to the Y1 amplifier (preamplifier), and the push-pull output signals ② and ③ are sent to the Y2 amplifier after being delayed by Г1 time in the delay stage. After amplification, sufficiently large signals ④ and ⑤ are generated and added to the Y-axis deflection plate of the oscilloscope. In order to display a complete and stable waveform on the screen, the measured signal ③ of the Y-axis is introduced into the trigger circuit of the X-axis system, and a trigger pulse ⑥ is generated at a certain level value of the positive (or negative) polarity of the introduced signal, which starts the sawtooth wave scanning circuit (time base generator) and generates a 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 greater 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. The z-axis system is used to amplify the positive scanning voltage and convert it into a positive rectangular wave and send it to the oscilloscope grid. This makes the waveform displayed in the positive scanning process have a certain fixed brightness, and smears in the return scanning process.

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

An oscilloscope often contains a precise and stable square wave signal generator for calibrating the oscilloscope.

03Oscilloscope
use

This section introduces how to use an oscilloscope. There are many types and models of oscilloscopes, and their functions are also different. In digital circuit experiments, 20MHz or 40MHz dual-trace oscilloscopes are more commonly used. The usage of these oscilloscopes is similar. This section does not target a specific model of oscilloscope, but only introduces the common functions of oscilloscopes in digital circuit experiments from a conceptual perspective.

1. Fluorescent screen