The principle and application of oscilloscope

　　In digital circuit testing, it is necessary to use several instruments and instruments to observe the test phenomena and results. Commonly used electronic measuring instruments include multimeters, logic pens, ordinary oscilloscopes, storage oscilloscopes, logic analyzers, etc. The use of multimeters and logic pens is relatively simple, while logic analyzers and storage oscilloscopes are not widely used in digital circuit education experiments. Oscilloscopes are a very widely used instrument with relatively complex use. This chapter introduces the principles and use of oscilloscopes from the perspective of application.

1 Working principle of oscilloscope
　　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, which are displayed on a fluorescent screen for measurement. It is an essential instrument for observing digital circuit test phenomena, analyzing test problems, and measuring test 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
　　Cathode ray tube (CRT), also known as oscilloscope, is the basis of 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 the oscilloscope

　　2) Fluorescent
　　screen The screen of today's oscilloscopes is usually a rectangular plane, with a layer of phosphorescent material deposited on the inside and outside 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 conducive to improving the brightness of the bright spot. The aluminum film also has other functions such as heat dissipation.

　　After electrons are bombarded continuously, the bright spot cannot disappear immediately but needs to be retained for a period of time. The time it takes for the brightness of the bright spot to drop to 10% of the initial 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 more than 1s is very long afterglow. Ordinary oscilloscopes are equipped with medium afterglow oscilloscope tubes, high-frequency oscilloscopes use short afterglow, and low-frequency oscilloscopes use long afterglow.

　　Because the phosphorescent materials used are different, different colors of light can be emitted on the screen. Usually, oscilloscopes use oscilloscope tubes that emit green light to protect people's eyes.

　　3) Electron gun and its control
　　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 fine 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 sleeved on the outside of the cathode. Because the grid potential is lower than that of the cathode, it controls the electrons emitted by the cathode. Usually, 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. The W1 potentiometer in the control circuit can change the grid potential, control the electron flow density directed to the fluorescent screen, and thus reach the brightness of the control 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 rush toward the fluorescent screen.

　　The electron beam goes through two focusing processes in the process of running from the cathode to the fluorescent screen. The first focusing is completed by K, G1, G2, K, K, G1, G2 are called the first electron lens of the oscilloscope. The second focusing occurs in the G2, A1, A2 area. Adjusting the potential of the second anode A2 can make the electron beam just focus 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 collecting electrode. Sometimes adjusting the A1 voltage still cannot meet the excellent focusing requirements, and it is necessary to fine-tune the voltage of the second anode A2, which is also called the auxiliary collecting electrode.

　　4) Deflection system
　　The deflection system controls the direction of the electron beam so that the light spot on the screen describes the waveform of the measured signal as the external signal changes. In Figure 8.1, two pairs of perpendicular deflection plates Y1, Y2 and X1, X2 form a deflection system. The Y-axis deflection plate is in front and the X-axis deflection plate is in the back, so the Y-axis has high sensitivity (the measured signal is added to the Y-axis after processing). Voltages are applied to the two pairs of deflection plates separately, 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.

　　5) Power supply of oscilloscope
　　In order for the oscilloscope to work properly, there are certain requirements for the power supply. It is required 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~-10V), and it is adjustable to achieve brightness adjustment. The first anode is positive (about +100V~+600V), which should also be adjustable for concentration adjustment. The second anode is connected to the front accelerating electrode, and is positive high voltage (about +1000V) to the cathode, and the adjustable range relative to the ground potential is ±50V. Because the current of each electrode of the oscilloscope is very small, it can be powered by a common high voltage through a resistor divider.

Figure 2 Oscilloscope basic structure block diagram

　　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 expander (pre-expansion), and the push-pull output signals ② and ③ are sent to the Y2 expander after a delay of Г1 time. After expansion, 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, and the sawtooth scanning circuit (time base generator) is initiated to generate a scanning voltage ⑦. Because there is a time delay Г2 from triggering to initiating 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 expanded by the X-axis expander, and push-pull outputs ⑨ and ⑩ are generated, which are added to the X-axis deflection plate of the oscilloscope. The z-axis system is used to expand the positive range of the scanning voltage, and it is converted into a positive rectangular wave and sent to the gate of the oscilloscope. This makes the waveform flashing on the forward scanning path have a certain fixed brightness, while smearing occurs on the return scanning path.

　　The above is the basic working principle of the oscilloscope. Dual-trace display uses an electronic switch to display two different measured signals input by the Y axis on the fluorescent screen separately. Due to the visual persistence of the human eye, when the switching frequency reaches a certain level, two stable and clear signal waveforms are seen. The
　　oscilloscope usually has an accurate and stable square wave signal generator for calibrating the oscilloscope.

2. Oscilloscope Application
　　This section introduces the application of oscilloscopes. There are many types and models of oscilloscopes with different functions. The 20MHz or 40MHz dual-trace oscilloscopes are more commonly used in digital circuit tests. These oscilloscopes have very different uses. This section is not about a certain type of oscilloscope, but only introduces the common functions of oscilloscopes in digital circuit tests from a conceptual perspective.

　　1) Fluorescent screen
　　The fluorescent screen is the display part of the oscilloscope. 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, and 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%, 10% and other symbols, and the horizontal direction is marked with 10%, 90% symbols, which are used to measure parameters such as DC level, AC signal fluctuation, delay time, etc. The voltage value and time value can be obtained by multiplying the number of grids occupied by the measured signal on the screen by the appropriate proportional constant (V/DIV, TIME/DIV).