In the experiment of digital circuits, several instruments and meters need to be used to observe the experimental phenomena and results. Commonly used electronic measuring instruments include multimeters, logic pens, ordinary oscilloscopes, storage oscilloscopes, and logic analyzers. The use of multimeters and logic pens is relatively simple, but logic analyzers and storage oscilloscopes are currently not widely used in digital circuit teaching experiments. Oscilloscope is a very widely used and relatively complex instrument. This chapter introduces the principle and usage of the oscilloscope from the perspective of use.
1 Working principle of an oscilloscope An oscilloscope is an electronic measuring instrument that uses the characteristics of an electronic oscilloscope to convert an alternating electrical signal that cannot be directly observed by the human eye into an image and displays it on a fluorescent screen for measurement. It is an indispensable and important instrument for observing experimental phenomena of digital circuits, analyzing problems in experiments, and measuring experimental results. The oscilloscope consists of an oscilloscope tube and a power supply system, a synchronization system, an X-axis deflection system, a Y-axis deflection system, a delayed scanning system, and a standard signal source.
1.1 Oscilloscope Tube The cathode ray tube (CRT) is short for oscilloscope tube, which is the core of the oscilloscope. It converts electrical signals into optical signals. The three parts of the electron gun, the deflection system and the 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, and a phosphor film is deposited on the inner surface of a phosphorescent material. 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 bright spots. The aluminum film has an internal reflection effect, which is beneficial to increase the brightness of the bright spot. Aluminum film also has other functions such as heat dissipation.
When the electrons stop bombardment, the bright spots cannot disappear immediately but must be retained for a period of time. The time after which the brightness of the bright spot drops to 10% of the original value is called "afterglow time". Afterglow time shorter than 10μs is very short afterglow, 10μs-1ms is short afterglow, 1ms-1. General oscilloscopes are equipped with mid-afterglow oscilloscopes, 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 a green oscilloscope to protect people's eyes.
2. The electron gun and focusing electron gun are composed of filament (F), cathode (K), grid (G1), front accelerator (G2) (or second grid), first anode (A1) and second anode (A2) . Its role is to emit electrons and form a very thin high-speed electron beam. The filament is energized to heat the cathode, and the cathode is heated to emit electrons. The grid is a metal cylinder with a small hole on the top, which is sheathed outside the cathode. Since the grid potential is lower than the cathode, it controls the electrons emitted from the cathode. Generally, only a small amount of electrons with a large initial velocity of movement can pass through the small holes of the grid and run toward the fluorescent screen under the action of the anode voltage. The electrons with 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 electron current density directed to the fluorescent screen, thereby adjusting the brightness of the bright spot. The first anode, the second anode, and the front accelerating pole 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 cathode electrons towards the phosphor screen.
In the process of the electron beam running from the cathode to the fluorescent screen, it undergoes two focusing processes. The first focusing is done 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 areas. Adjusting the potential of the second anode A2 can cause the electron beam to converge to a point on the phosphor screen. This is the second focusing. The voltage on A1 is called the focus voltage, and A1 is also called the focus pole. Sometimes adjusting the voltage of A1 still does not satisfy 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 beams, so that the light spot on the fluorescent screen draws the waveform of the measured signal with the change of the applied signal. In Figure 8.1, Y1, Y2 and X1, X2 two pairs of perpendicular deflection plates 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). Two pairs of deflection plates are respectively applied with voltage, so that an electric field is formed between the two pairs of deflection plates, and the electron beams are respectively controlled to deflect in the vertical direction and the horizontal direction.
4. Oscilloscope power supply In order to make the oscilloscope work normally, there are certain requirements for the power supply. It is specified 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 work 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 is a positive potential (about + 100V ~ + 600V), which should also be adjustable for focus adjustment. The second anode is connected to the front accelerating pole and has a positive high voltage (about + 1000V) to the cathode, 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 The basic composition of the 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 graphic shape 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 applied to the X-axis deflection plate of the oscilloscope and the measured signal is added to the y-axis (after proportional enlargement or reduction), the oscilloscope screen will display the measured The graph of the signal over time. Among electrical signals, a signal that is proportional to the time variable over a period of time is a sawtooth wave.
The basic block diagram of the oscilloscope. It consists of five parts: oscilloscope, Y-axis system, X-axis system, Z-axis system and power supply.
The measured signal ①is connected to the “Y†input terminal, and is appropriately attenuated by the Y-axis attenuator and then sent to the Y1 amplifier (pre-amplification), and the push-pull output signals ② and ③. After the delay stage delay Г1 time, to Y2 amplifier. After amplification, the signals ④ and ⑤ are large enough to be added to the Y-axis deflection plate of the oscilloscope. In order to display a complete stable waveform on the screen, the Y-axis measured signal ③ 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 to start Sawtooth wave scanning circuit (time base generator) generates scanning voltage ⑦. Since there is a time delay Г2 from triggering to start scanning, in order to ensure that the X-axis starts scanning before the Y-axis signal reaches the fluorescent screen, the Y-axis delay time Г1 should be slightly larger than the X-axis delay time Г2. 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 path of the scanning voltage, and it becomes a positive rectangular wave, which is sent to the grid of the oscilloscope. This makes the waveform displayed on the front of the scan have a certain brightness, and erases on the back of the scan.
The above is the basic working principle of the oscilloscope. The dual-track display uses an electronic switch to display two different measured signals input on the Y axis on the fluorescent screen. Due to the persistence of the human eye's vision, when the conversion frequency is high to a certain degree, two stable and clear signal waveforms are seen.
There is often an accurate and stable square wave signal generator in the oscilloscope, which is used to verify the oscilloscope.
2 Oscilloscope use This section introduces how to use the oscilloscope. There are many types and models of oscilloscopes with different functions. In the digital circuit experiment, the dual trace oscilloscope with 20MHz or 40MHz is used more. The usage of these oscilloscopes is similar. This section does not target a certain type of oscilloscope, but only introduces the common functions of the oscilloscope in digital circuit experiments conceptually.
2.1 Fluorescent screen The fluorescent screen is the display part of the oscilloscope. There are multiple scale lines on 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 divisions, the vertical direction is divided into 8 divisions, and each division is divided into 5 divisions. The vertical direction is marked with 0%, 10%, 90%, 100% and other signs, and the horizontal direction is marked with 10% and 90% signs, which are used for measuring DC level, AC signal amplitude, delay time and other parameters. The voltage value and time value can be obtained by multiplying the proportion of the measured signal on the screen by the appropriate proportional constant (V / DIV, TIME / DIV).
2.2 Oscilloscope tube and power supply system Power
Oscilloscope main power switch. When this switch is pressed, the power indicator lights, indicating that the power is on.
2. Intensity
Rotating this knob can change the brightness of the light spot and scanning line. It can be smaller when observing low-frequency signals, and larger when high-frequency signals.
Generally, it should not be too bright to protect the fluorescent screen.
3. Focus
The focus knob adjusts the cross-sectional size of the electron beam to focus the scanning line to the clearest state.
4. Illuminance
This knob adjusts the brightness of the light behind the fluorescent screen. Under normal indoor light, the lighting is darker. In an environment with insufficient indoor lighting, the lighting can be properly adjusted.
2.3 Vertical deflection factor and horizontal deflection factor Vertical deflection factor selection (VOLTS / DIV) and fine adjustment Under the action of the unit input signal, the distance that the light spot shifts on the screen is called the shift sensitivity. This definition applies to both the X-axis and Y-axis. The reciprocal of 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 the customary usage and the convenience of measuring the voltage reading, sometimes the deflection factor is also regarded as the sensitivity.
Each channel in the trace oscilloscope has a vertical deflection factor selection band switch. Generally, it is divided into 10 gears from 5mV / DIV to 5V / DIV according to 1, 2, 5 ways. The value indicated by the band switch represents the voltage value in the vertical direction on the screen. For example, when the band switch is set to 1V / DIV, if the signal light spot moves one grid on the screen, it means that the input signal voltage changes by 1V.
There is often a small knob on each band switch to fine-tune each vertical deflection factor. Turn it clockwise to the end, in the "calibration" position, at this time the vertical deflection factor value is consistent with the value indicated by the band switch. Turn this knob counterclockwise to fine-tune the vertical deflection factor. After fine-tuning the vertical deflection factor, it will cause inconsistency with the indication 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 increases several times (the deflection factor decreases several times). For example, if the deflection factor indicated by the band switch is 1V / DIV, when the × 5 expansion state is used, 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 determine the voltage value of the measured signal.
2. Time base selection (TIME / DIV) and fine adjustment The use of time base selection and fine adjustment is similar to vertical deflection factor selection and fine adjustment. Time base selection is also realized by a band switch, and the time base is divided into several gears according to 1, 2, and 5 modes. The indicated value of the band switch represents the time value when the light spot moves one grid in the horizontal direction. For example, in the 1μS / DIV file, the light spot moves one grid on the screen to represent 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 by the band switch. Turn the knob counterclockwise to fine-tune the time base. After the knob is pulled out, it is in the scan expansion state. It is usually × 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 file, the time value represented by a horizontal grid on the screen in the extended scan state is equal to 2μS × (1/10) = 0.2μS
There are clock signals of 10MHz, 1MHz, 500kHz, and 100kHz on the TDS test bench, which are generated by the quartz crystal oscillator and frequency divider. The accuracy is very high and can be used to calibrate the time base of the oscilloscope.
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 COS5041 oscilloscope provides a square wave signal with VP-P = 2V and f = 1kHz.
The Position knob on the front panel of the oscilloscope adjusts the position of the signal waveform on the fluorescent screen. Turn the horizontal displacement knob (marked with a horizontal bidirectional arrow) to move the signal waveform left and right, and turn the vertical displacement knob (marked with a vertical bidirectional 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 options for input channels: 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 channels are selected, the oscilloscope displays both channel 1 and channel 2 signals. When testing signals, first connect the ground of the oscilloscope with the ground of the circuit under test. According to the selection of the input channel, plug the oscilloscope probe into the corresponding channel socket. The ground on the oscilloscope probe is connected to the ground of the circuit under test. The oscilloscope probe contacts the point under test. There is a two-position switch on the oscilloscope probe. When this switch is set 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 set 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 options for input coupling mode: alternating current (AC), ground (GND), direct current (DC). When "Ground" is selected, the scan line shows the location of the "oscilloscope ground" on the phosphor screen. DC coupling is used to determine the absolute value of the signal DC and observe very low frequency signals. AC coupling is used to observe AC and AC signals containing DC components. In the digital circuit experiment, generally choose the "DC" mode in order to observe the absolute voltage value of the signal.
2.5 Trigger Section 1 points out that after the measured signal is input from the Y axis, a part is sent to the Y axis deflection plate of the oscilloscope, driving the light spot to move in a vertical direction on the fluorescent screen in proportion; the other part is shunted to the x axis deflection system Generate a trigger pulse, trigger a scan generator, generate a repeated sawtooth wave voltage, and apply it to the X deflection plate of the oscilloscope to move the light spot in the horizontal direction. The two are united. Test signal graphics. It can be seen that the correct triggering method directly affects the effective operation of the oscilloscope. In order to get a stable and clear signal waveform on the fluorescent screen, it is very important to master the basic trigger function and its operation method.
1. Trigger source (Source) selection To make the screen display a stable waveform, you need to add the signal under test itself or a certain time relationship with the signal under test to the trigger circuit. Trigger source selection determines where the trigger signal is supplied. There are usually three trigger sources: internal trigger (INT), power trigger (LINE), external trigger EXT).
Internal triggering uses the measured signal as the trigger signal, which is a frequently used triggering method. Since the trigger signal itself is part of the signal under test, a very stable waveform can be displayed on the screen. Either channel 1 or channel 2 of the dual trace oscilloscope can be selected as the trigger signal.
The power trigger uses the AC power frequency signal as the trigger signal. This method is effective when measuring signals related to AC power frequency. Especially it is more effective when measuring the low-level AC noise of audio circuits and thyristors.
The external trigger uses an external signal as the trigger signal, and the external signal 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 signal under test is not used as a trigger signal, it does not matter when the signal starts to be scanned.
Correct selection of the trigger signal is very important to 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 select the internal trigger. For a signal with a complex period and there is a signal with a periodic relationship, it may be more preferable to select the external trigger. it is good.
2. Trigger coupling (Coupling) mode selection There are many ways of coupling the trigger signal to the trigger circuit, the purpose is to stabilize and reliable the trigger signal. Here are a few commonly used.
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 DC components are not considered to form a stable trigger. However, if the frequency of the trigger signal is less than 10 Hz, it will cause difficulty in triggering.
DC coupling (DC) does not isolate 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 low-frequency suppression (LFR) triggers, the trigger signal is added to the trigger circuit through the high-pass filter, and the low-frequency component of the trigger signal is suppressed; when high-frequency suppression (HFR) trigger, the trigger signal is added to the trigger circuit through the low-pass filter, the trigger signal High frequency components are suppressed. In addition, there is TV synchronization (TV) triggering for TV maintenance. Each of these trigger coupling methods has its own scope of application and needs to be experienced in use.
3. Trigger Level (Level) and Trigger Polarity (Slope)
Trigger level adjustment is also called synchronous adjustment, which makes the scanning synchronized with the signal under test. 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 sweep 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 cannot be triggered steadily with the level knob, use the Hold Off knob to adjust the holdoff time (scan pause time) of the waveform to enable stable synchronization of the sweep and the waveform.
The polarity switch is used to select the polarity of the trigger signal. When dialed in the "+" position, in the direction of signal increase, a trigger is generated when the trigger signal exceeds the trigger level. When dialed in the "-" position, in the direction of signal reduction, a trigger is generated when the trigger signal exceeds the trigger level. The trigger polarity and trigger level determine the trigger point of the trigger signal.
2.6 Scan Mode (SweepMode)
There are three scanning modes: Auto (Auto), Normal (Norm) and Single (Single).
Automatic: When there is no trigger signal input, or when the trigger signal frequency is lower than 50Hz, the scan is self-excited.
Normal state: 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: Single button is similar to reset switch. In the single scan mode, the scan circuit is reset when the single button is pressed, and the Ready light turns on at this time. A scan is generated after the trigger signal arrives. After the single scan, prepare to turn off the light. A single sweep is used to observe non-periodic signals or single transient signals, and it is often necessary to take pictures of the waveform.
The above briefly introduces the basic functions and operations of the oscilloscope. The oscilloscope has some more complicated functions, such as delayed sweep, trigger delay, XY working mode, etc., which will not be introduced here. Getting started with an oscilloscope is easy, and you must master it in your application if you are really skilled. It is worth pointing out that although the oscilloscope has more functions, in many cases it is better to use other instruments and meters. 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; it is better to use a logic analyzer when measuring the single pulse pulse width.
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