The principle and use of oscilloscope

The principle and use of oscilloscope

The composition of the oscilloscope In digital circuit experiments, several instruments and meters are needed to observe experimental 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 yet widely used in digital circuit teaching experiments. The oscilloscope is a very widely used and relatively complex instrument. This chapter introduces the principle and use of the oscilloscope from the perspective of use.
1 Working principle of oscilloscope The oscilloscope is an electronic measuring instrument that uses the characteristics of electronic oscilloscopes to convert alternating electrical signals that cannot be directly observed by the human eye into images and displays them on the fluorescent screen for measurement. It is an important instrument that is indispensable for observing digital circuit experimental phenomena, analyzing problems in experiments, and measuring experimental results. The oscilloscope consists of an oscilloscope and a power supply system, a synchronization system, an X-axis deflection system, a Y-axis deflection system, a delay scanning system, and a standard signal source. 1.1 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. 1. Fluorescent screen The current oscilloscope screen is usually a rectangular plane, 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, 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 electrons stop bombarding, the bright spot cannot disappear immediately but remains for a period of time. The time it takes for the brightness of the bright spot to drop to 10% of the original value is called "afterglow time". Afterglow time shorter than 10μs is extremely short afterglow, 10μs-1ms is short afterglow, 1ms-0.1s is medium afterglow, 0.1s-1s is long afterglow, and greater than 1s is extremely 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 on 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 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 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. The W1 potentiometer in the adjustment circuit can change the grid potential and 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. In the process of the electron beam rushing from the cathode to the fluorescent 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 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 good focusing requirements, and it is necessary to fine-tune the voltage of the second anode A2. 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 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 Xl, 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). 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, respectively controlling the deflection of the electron beam in the vertical and horizontal directions. 4. Power supply of oscilloscope In order for the oscilloscope to work normally, 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 a negative potential relative to the cathode (-30V~-100V), and it is adjustable to achieve brightness adjustment. The first anode is a positive potential (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 is very small, it can be powered by a common high voltage through a resistor divider. 1.2 Basic components of an oscilloscope From the previous section, it can be seen that 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 amplification or reduction) is added to the y-axis, the graph of the measured signal changing with time will be displayed on the oscilloscope screen. In the electrical signal, 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: an oscilloscope, a Y-axis system, an X-axis system, a Z-axis system, and a power supply. 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 a delay of Г1 time. 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, which is sent to the gate of the oscilloscope. This makes the waveform displayed on the positive scanning path have a certain fixed brightness, and smears on the return scanning path. The above is the basic working principle of the oscilloscope. The dual-trace display uses an electronic switch to display the two different measured signals input by the Y-axis on the fluorescent screen respectively. 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. The oscilloscope often has an accurate and stable square wave signal generator for calibrating the oscilloscope. 2 How to use the oscilloscope





















This section introduces how to use an oscilloscope. 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 experiments. 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.
2.1 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, 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 signs, and the horizontal direction is marked with 10%, 90% signs, which are used to measure parameters such as DC level, AC signal amplitude, and delay time. 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).
2.2 Oscilloscope tube and power supply system
1. Power (Power)
Oscilloscope main power switch. When this switch is pressed, the power indicator light is on, indicating that the power is on.
2. Brightness (Intensity)
Rotating this knob can change the brightness of the light spot and the scan line. It can be smaller when observing low-frequency signals and larger when observing high-frequency signals.
Generally, it should not be too bright to protect the fluorescent screen.
3. Focus (Focus)
The focus knob adjusts the size of the electron beam cross-section to focus the scan line to the clearest state.
4. Scale brightness (Illuminance)
This knob adjusts the brightness of the illuminance behind the fluorescent screen. Under normal indoor light, the illuminance is better. In an environment with insufficient indoor light, the illuminance can be appropriately brightened. 2.3 Vertical deflection factor and horizontal deflection factor 1. Vertical deflection factor selection (VOLTS/DIV) and fine adjustment The distance that the light spot deflects on the screen under the action of a unit input signal is called the offset sensitivity. This definition applies to both the X-axis and the Y-axis. The reciprocal of the sensitivity is called the deflection factor. The unit of vertical sensitivity is cm/V, cm/mV or DIV/mV, DIV/V, and the unit of vertical deflection factor is V/cm, mV/cm or V/DIV, mV/DIV. In fact, due to customary usage and the convenience of measuring voltage readings, the deflection factor is sometimes regarded as sensitivity. Each channel in the tracer oscilloscope has a vertical deflection factor selection band switch. Generally, it is divided into 10 levels from 5mV/DIV to 5V/DIV in the form of 1, 2, 5. The value indicated by the band switch represents the voltage value of one grid in the vertical direction on the fluorescent screen. For example, when the band switch is set to 1V/DIV, if the signal light spot on the screen moves one grid, it means that the input signal voltage changes by 1V. There is often a small knob on each band switch to fine-tune the vertical deflection factor of each level. Turn it clockwise to the bottom, which is in the "calibration" position. At this time, the vertical deflection factor value is consistent with the value indicated by the band switch. Rotate this knob counterclockwise to fine-tune the vertical deflection factor. After the vertical deflection factor is fine-tuned, it will cause inconsistency with the indicated value of the band switch, which should be paid attention to. Many oscilloscopes have a vertical expansion function. When the fine-tuning knob is pulled out, the vertical sensitivity is expanded several times (the deflection factor is reduced several times). For example, if the deflection factor indicated by the band switch is 1V/DIV, when the ×5 expansion state is adopted, the vertical deflection factor is 0.2V/DIV. When doing digital circuit experiments, the ratio of the vertical movement distance of the measured signal on the screen to the vertical movement distance of the +5V signal is often used to judge the voltage value of the measured signal. 2. Time base selection (TIME/DIV) and fine-tuning The use of time base selection and fine-tuning is similar to that of vertical deflection factor selection and fine-tuning. Time base selection is also realized through a band switch, and the time base is divided into several gears according to 1, 2, and 5. The indicated value of the band switch represents the time value of the light spot moving one grid in the horizontal direction. For example, in the 1μS/DIV gear, the light spot moving one grid on the screen represents the time value of 1μS. The "fine adjustment" knob is used for time base calibration and fine adjustment. When it is turned clockwise to the calibration position, the time base value displayed on the screen is consistent with the nominal value shown on the band switch. Turn the knob counterclockwise to fine-tune the time base. When the knob is pulled out, it is in the scanning expansion state. Usually it is ×10 expansion, that is, the horizontal sensitivity is expanded by 10 times and the time base is reduced to 1/10. For example, in the 2μS/DIV gear, the time value represented by one horizontal grid on the fluorescent screen in the scanning expansion state is equal to 2μS×(1/10)=0.2μS . The standard signal source CAL of the oscilloscope is specially used to calibrate the time base and vertical deflection factor of the oscilloscope. For example, the standard signal source of the COS5041 oscilloscope provides a square wave signal with VP-P=2V, f=1kHz. The displacement (Position) knob on the front panel of the oscilloscope adjusts the position of the signal waveform on the fluorescent screen. Rotate the horizontal displacement knob (marked with a horizontal double-headed arrow) to move the signal waveform left and right, and rotate the vertical displacement knob (marked with a vertical double-headed arrow) to move the signal waveform up and down. 2.4 Input channel and input coupling selection 1. Input channel selection There are at least three input channel selection modes: channel 1 (CH1), channel 2 (CH2), and dual channel (DUAL). When channel 1 is selected, the oscilloscope only displays the signal of channel 1. When channel 2 is selected, the oscilloscope only displays the signal of channel 2. When dual channel is selected, the oscilloscope displays the signal of channel 1 and the signal of channel 2 at the same time. When testing the signal, first connect the ground of the oscilloscope to the ground of the circuit under test. According to the selection of the input channel, plug the oscilloscope probe into the corresponding channel socket, connect the ground on the oscilloscope probe to the ground of the circuit under test, and the oscilloscope probe touches the measured point. There is a two-position switch on the oscilloscope probe. When this switch is turned to the "×1" position, the measured signal is sent to the oscilloscope without attenuation, and the voltage value read from the fluorescent screen is the actual voltage value of the signal. When this switch is turned to the "×10" position, the measured signal is attenuated to 1/10 and then sent to the oscilloscope. The voltage value read from the fluorescent screen multiplied by 10 is the actual voltage value of the signal. 2. Input coupling mode There are three input coupling modes: AC, GND, and DC. When "GND" is selected, the scan line shows the position of the "oscilloscope ground" on the screen. DC coupling is used to measure the DC absolute value of the signal and observe very low frequency signals. AC coupling is used to observe AC and AC signals containing DC components. In digital circuit experiments, the "DC" mode is generally selected to observe the absolute voltage value of the signal. 2.5 Trigger



















The first section points out that after the measured signal is input from the Y axis, a part of it is sent to the Y axis deflection plate of the oscilloscope, driving the light spot to move in the vertical direction on the fluorescent screen in proportion; the other part is diverted to the x axis deflection system to generate a trigger pulse, trigger the scanning generator, generate a repeated sawtooth wave voltage and add it to the x deflection plate of the oscilloscope, so that the light spot moves in the horizontal direction. The two are combined, and the figure drawn by the light spot on the fluorescent screen is the measured signal figure. It can be seen that the correct triggering method directly affects the effective operation of the oscilloscope. In order to obtain a stable and clear signal waveform on the fluorescent screen, it is very important to master the basic triggering function and its operation method.
1. Trigger source (Source) selection
To display a stable waveform on the screen, the measured signal itself or a trigger signal with a certain time relationship with the measured signal needs to be added to the trigger circuit. The trigger source selection determines where the trigger signal is supplied. There are usually three trigger sources: internal trigger (INT), power trigger (LINE), and external trigger EXT).
Internal trigger uses the measured signal as the trigger signal, which is a frequently used triggering method. Since the trigger signal itself is part of the measured signal, a very stable waveform can be displayed on the screen. In a dual-trace oscilloscope, either channel 1 or channel 2 can be selected as the trigger signal.
Power triggering uses the AC power frequency signal as the trigger signal. This method is effective when measuring signals related to the AC power frequency. It is especially effective when measuring low-level AC noise of audio circuits and thyristors.
External triggering uses an external signal as the trigger signal, which is input from the external trigger input terminal. There should be a periodic relationship between the external trigger signal and the measured signal. Since the measured signal is not used as a trigger signal, when to start scanning has nothing to do with the measured signal.
Correctly selecting the trigger signal has a great relationship with the stability and clarity of the waveform display. For example, in the measurement of digital circuits, for a simple periodic signal, it may be better to choose an internal trigger, while for a signal with a complex period and a signal with a periodic relationship with it, it may be better to choose an external trigger.
2. Trigger coupling (Coupling) mode selection
There are many ways to couple the trigger signal to the trigger circuit, the purpose is to make the trigger signal stable and reliable. Here are several commonly used ones.
AC coupling is also called capacitive coupling. It only allows triggering with the AC component of the trigger signal, and the DC component of the trigger signal is blocked. This coupling method is usually used when the DC component is not considered to form a stable trigger. However, if the frequency of the trigger signal is less than 10Hz, it will cause difficulty in triggering.
DC coupling (DC) does not block the DC component of the trigger signal. When the frequency of the trigger signal is low or the duty cycle of the trigger signal is large, it is better to use DC coupling.
When the low frequency suppression (LFR) triggers, the trigger signal is added to the trigger circuit through a high-pass filter, and the low frequency component of the trigger signal is suppressed; when the high frequency suppression (HFR) triggers, the trigger signal is added to the trigger circuit through a low-pass filter, and the high frequency component of the trigger signal is suppressed. In addition, there is a TV synchronization (TV) trigger for TV maintenance. These trigger coupling methods have their own scope of application, which needs to be experienced in use.
3. Trigger level (Level) and trigger polarity (Slope)
Trigger level adjustment is also called synchronization adjustment, which synchronizes the scan with the measured signal. The level adjustment knob adjusts the trigger level of the trigger signal. Once the trigger signal exceeds the trigger level set by the knob, the scan is triggered. Turn the knob clockwise to increase the trigger level; turn the knob counterclockwise to decrease the trigger level. When the level knob is adjusted to the level lock position, the trigger level is automatically kept within the amplitude of the trigger signal, and a stable trigger can be generated without level adjustment. When the signal waveform is complex and the level knob cannot be used to stably trigger, use the Hold Off knob to adjust the waveform holdoff time (scan pause time) to synchronize the scan with the waveform.
The polarity switch is used to select the polarity of the trigger signal. When the dial is in the "+" position, a trigger is generated when the trigger signal exceeds the trigger level in the direction of signal increase. When the dial is in the "-" position, a trigger is generated when the trigger signal exceeds the trigger level in the direction of signal decrease. The trigger polarity and trigger level jointly determine the trigger point of the trigger signal.
2.6 Sweep Mode
Sweep has three sweep modes: Auto, Norm, and Single.
Auto: When there is no trigger signal input or the trigger signal frequency is lower than 50Hz, the scan is self-excited.
Normal: When there is no trigger signal input, the scan is in the ready state and there is no scan line. After the trigger signal arrives, the scan is triggered.
Single: The single button is similar to a reset switch. In the single scan mode, the scan circuit is reset when the single button is pressed, and the Ready light is on. A scan is generated after the trigger signal arrives. After the single scan is completed, the Ready light goes out. Single scan is used to observe non-periodic signals or single transient signals, and it is often necessary to take a picture of the waveform.
The above briefly introduces the basic functions and operations of the oscilloscope. The oscilloscope also has some more complex functions, such as delayed scan, trigger delay, XY working mode, etc., which are not introduced here. It is easy to get started with the oscilloscope, but you need to master it in application to be truly proficient. It is worth pointing out that although the oscilloscope has many functions, it is better to use other instruments and meters in many cases. For example, in a digital circuit experiment, it is much simpler to use a logic pen to determine whether a single pulse with a narrow pulse width occurs; when measuring the pulse width of a single pulse, it is better to use a logic analyzer.