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1 The working principle of oscilloscope Oscilloscopes use the characteristics of the electronic oscilloscope, the human eye can not be directly observed alternating electrical signals into an image, displayed on a fluorescent screen in order to measure the electronic measurement instruments. It is an important instrument essential for observing the experimental phenomena of digital circuits, analyzing the problems in the experiment, and measuring the experimental results. Oscilloscope by the oscilloscope and power supply system, synchronization system, X-axis deflection system, Y-axis deflection system, delay scanning system, the standard signal source. 1.1 Oscilloscope
Cathode ray tube (CRT) referred to as the oscilloscope, is the core of the oscilloscope. It converts electrical signals into optical signals. Electron gun, deflection system and fluorescent screen three parts sealed in a vacuum glass shell, constituting a complete oscilloscope.
1. fluorescent screen
Now the oscilloscope screen is usually a rectangular plane, the inner surface of a layer of phosphorescent material deposited to form a fluorescent film. In the fluorescent film is often added to a layer of evaporated aluminum film. High-speed electrons through the aluminum film, the impact of phosphor and the formation of bright light. Aluminum film has the role of internal reflection, which is conducive to improving the brightness of the brightness. Aluminum film also has other functions such as heat dissipation.
When the electrons stop bombarding, the bright spot can not immediately disappear but to retain a period of time. Bright spot brightness down to 10% of the original value of the time elapsed is called "afterglow time". Afterglow time shorter than 10μs for the very short afterglow, 10μs-1ms for the short afterglow, 1ms-0.1s for the afterglow, 0.1s-1s for the long afterglow, greater than 1s for the very long afterglow. The general oscilloscope is equipped with an oscilloscope tube in the afterglow, high-frequency oscilloscope selection of short afterglow, low-frequency oscilloscope selection of long afterglow.
Due to the different phosphor materials used, the fluorescent screen can emit different colors of light. General oscilloscopes use more green light oscilloscope to protect the human eye. 2. Electron gun and focus
Electron gun by the filament (F), cathode (K), gate (G1), before the accelerating pole (G2) (or the second gate), the first anode (A1) and the second anode (A2). Its function is to emit electrons and form a very fine, high-speed electron beam. A filament is energized to heat the cathode, which is heated to emit electrons. The gate is a metal cylinder with a small hole at the top, set outside the cathode. As the gate potential than the cathode is low, the cathode emission of electrons play a controlling role, generally only a small number of electrons with a large initial velocity of movement, under the action of the anode voltage can pass through the gate hole, run to the fluorescent screen. Initial velocity of small electrons still return to the cathode. If the gate potential is too low, all the electrons return to the cathode, that is, the tube cutoff. Adjust the circuit of the W1 potentiometer, you can change the gate potential, control the density of electron flow to the fluorescent screen, so as to adjust the brightness of the brightness. The first anode, the second anode and the front accelerating pole are all three metal cylinders on the same axis as the cathode. Before the accelerating pole G2 and A2 connected to the potential than A1 high. G2 positive potential of the cathode electrons run to the fluorescent screen to accelerate the role.
Electron beam from the cathode to the fluorescent screen in the process, after two focusing process. The first focus by K, G1, G2 to complete, K, K, G1, G2 is called the first electron lens of the oscilloscope. The second focus occurs in the G2, A1, A2 region, adjust the potential of the second anode A2, can make the electron beam just converge on the fluorescent screen at a point, which is the second focus.A1 voltage is called the focusing voltage, A1 is also called the focusing pole. Sometimes adjust the A1 voltage still can not meet the good focus, need to fine-tune the voltage of the second anode A2, A2 is also called auxiliary focusing pole. 3. Deflection system
Deflection system to control the direction of the electron rays, so that the point of light on the fluorescent screen with the changes in the applied signal depicts the waveform of the signal being measured. Figure 8.1, Y1, Y2 and Xl, X2 two pairs of mutually perpendicular to the deflection plate composed of the deflection system. y-axis deflection plate in front of the x-axis deflection plate in the back, so the y-axis sensitivity is high (the measured signal is processed and added to the y-axis). Two pairs of deflector plates were added to the voltage, so that the two pairs of deflector plates between the formation of an electric field, respectively, to control the electron beam in the vertical direction and the horizontal direction of the deflection. 4. Oscilloscope power supply
In order to make the normal operation of the oscilloscope, the power supply has certain requirements. Provides for the second anode and the deflection plate between the potential is similar, the average potential of the deflection plate is zero or close to zero. The cathode must operate at a negative potential. The gate G1 is at a negative potential (-30V to -100V) relative to the cathode and is adjustable for glow adjustment. The first anode is at a positive potential (about +100V~+600V) and should also be adjustable for use as a focus adjustment. The second anode is connected to the front accelerating pole and is positive high voltage (about +1000V) to the cathode, with an adjustable range of ±50V relative to the ground potential.Since the current of each electrode of the oscilloscope is very small, it can be supplied with a public *** high voltage through the resistor voltage divider. 1.2 Basic Composition of Oscilloscope From the previous subsection, it can be seen that as long as the voltage on the X-axis deflection plate and 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 varies with time. Therefore, as long as the x-axis deflection plate in the oscilloscope with a voltage proportional to the time variable, in the y-axis coupled with the signal under test (after proportional amplification or reduction), the oscilloscope screen will display the measured signal with the time change of the graph. In electrical signals, the signal that is proportional to the time variable over a period of time is a sawtooth wave.
The basic composition of the oscilloscope block diagram shown in Figure 2. It consists of five parts such as oscilloscope, Y-axis system, X-axis system, Z-axis system and power supply.
Figure 2 basic components of the oscilloscope block diagram The measured signal ① received by the "Y" input, through the Y-axis attenuator appropriate attenuation sent to the Y1 amplifier (preamplifier), push-pull output signals ② and ③. The delay stage delay Г1 time, to Y2 amplifier. The amplification produces signals ④ and ⑤ large enough to be added to the Y-axis deflector plate of the oscilloscope. In order to display a complete and stable waveform on the screen, the Y-axis measured signal ③ is introduced into the trigger circuit of the X-axis system, which generates a trigger pulse ⑥ at a certain level value of the positive (or negative) polarity of the introduced signal to activate the sawtooth waveform scanning circuit (time base generator) and generate a scanning voltage ⑦. Since there is a time delay Г2 from triggering to starting scanning, in order to ensure that the Y-axis signal reaches the fluorescent screen before the X-axis starts scanning, 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 produce push-pull outputs ⑨ and ⑩, which are added to the X-axis deflector of the oscilloscope tube. z-axis system is used to amplify the positive range of the scanning voltage, and it is turned into a positive rectangular wave, which is fed to the gate of the oscilloscope tube. gate of the oscilloscope. This allows the waveform to be displayed with a fixed glow on the scan forward and smeared on the scan backward.
These are the basic principles of oscilloscopes. Dual-trace display is the use of electronic switches to the Y-axis input of the two different signals under test are displayed on the fluorescent screen. Due to the visual retention effect of the human eye, when the conversion frequency is high to a certain extent, see the two stable, clear signal waveforms.
Oscilloscopes often have a precise and stable square wave signal generator for calibration oscilloscope. 2 oscilloscope use This section introduces the use of oscilloscopes. Oscilloscopes are available in many types and models with different functions. Digital circuit experiments use more 20MHz or 40MHz dual-trace oscilloscopes. The usage of these oscilloscopes is similar. This section does not focus on a particular model of oscilloscope, but only conceptually introduces the common functions of oscilloscopes in digital circuit experiments. 2.1 Fluorescent Screen
The fluorescent screen is the display part of the oscilloscope. There are multiple tick marks on the screen in both the horizontal and vertical directions, indicating the relationship between voltage and time of the signal waveform. The horizontal direction indicates the time, and the vertical direction indicates the voltage. The horizontal direction is divided into 10 frames, and the vertical direction is divided into 8 frames, and each frame is further divided into 5 parts. Vertical direction is marked with 0%, 10%, 90%, 100% and other signs, horizontal direction is marked with 10%, 90% signs for measuring DC level, AC signal amplitude, delay time and other parameters. According to the measured signal on the screen accounted for the number of frames multiplied by the appropriate constant of proportionality (V / DIV, TIME / DIV) can be derived from the voltage value and time value. 2.2 Oscilloscope and power supply system
1. Power (Power)
Oscilloscope main power switch. When this switch is pressed, the power indicator lights up, indicating that the power supply is on.
2. Glow (Intensity)
Rotate this knob to change the brightness of the light points and scan lines. Observe the low-frequency signals can be smaller, high-frequency signals larger.
Generally should not be too bright to protect the fluorescent screen.
3. Focus (Focus)
Focus knob to adjust the size of the cross-section of the electron beam, the scanning line will be focused into the clearest state.
4. Scale brightness (Illuminance)
This knob regulates the brightness of the illumination behind the fluorescent screen. Under normal indoor light, the illumination lamp dimmer good. In the environment of insufficient indoor light, the lighting can be adjusted appropriately. 2.3 Vertical Deflection Factor and Horizontal Deflection Factor
1. Vertical Deflection Factor Selection (VOLTS/DIV) and Fine Adjustment
The distance that the point of light is deflected from the screen under the action of a unit of the input signal is known as the offset sensitivity, a definition that applies to both the X and Y axes. 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, vertical deflection factor is V/cm, mV/cm or V/DIV, mV/DIV. In fact, because of the customary method and the convenience of measuring voltage readings, and sometimes the deflection factor as sensitivity.
Each channel of the tracking oscilloscope has a vertical deflection factor selection band switch. Generally, the band switch is divided into 10 steps from 5mV/DIV to 5V/DIV in 1, 2, and 5 ways. The value indicated by the band switch represents the voltage value of a vertical grid on the fluorescent screen. For example, if the band switch is placed at 1V/DIV, if the signal spot on the screen moves one frame, it represents a 1V change in the input signal voltage.
There is also a small knob on each band switch to fine-tune the vertical deflection factor for each step. Turn it clockwise to the end, in the "calibration" position, this time the vertical deflection factor value and the value indicated by the band switch. Turn this knob counterclockwise to fine-tune the vertical deflection factor. It should be noted that the fine adjustment of the vertical deflection factor may cause inconsistency with the value indicated by the band switch. Many oscilloscopes have a vertical expansion function that expands the vertical sensitivity by a factor of several (and reduces the deflection factor by a factor of several) when the trim knob is pulled out. For example, if the band switch indicates that the deflection factor is 1V/DIV, using the ×5 expansion state, the vertical deflection factor is 0.2V/DIV.
In the digital circuit experiments, the measured signal on the screen is often used to determine the vertical distance of the measured signal and the vertical distance of the signal +5V the ratio of the voltage value of the signal under test.
2. Time base selection (TIME / DIV) and fine-tuning
The use of time base selection and fine-tuning and vertical deflection factor selection and fine-tuning is similar. The time base selection is also realized by a band switch, which divides the time base into a number of steps in the manner of 1, 2, and 5. The indication value of the band switch represents the time value for the point of light to move horizontally by one frame. For example, in the 1μS/DIV mode, the point of light moves one frame on the screen to represent the time value of 1μS.
The "Fine Adjustment" knob is used for time base calibration and fine adjustment. When rotated clockwise to the bottom in the calibration position, the time base value displayed on the screen is the same as the nominal value shown by the band switch. Turning the knob counterclockwise fine tunes the time base. The knob is in the scanning extension state when it is pulled out. Usually ×10 expansion, that is, the horizontal sensitivity expanded by 10 times, the time base is reduced to 1 / 10. For example, in the 2μS / DIV gear, the scanning expansion of the state of the fluorescent screen in the horizontal grid represents the time value is equal to
2μS × (1 / 10) = 0.2μS
The TDS experimental stage has 10MHz, 1MHz, 500kHz, 100kHz clock signals, generated by a quartz crystal oscillator and frequency divider, are highly accurate and can be used to calibrate the oscilloscope's time base.
The oscilloscope's standard signal source, CAL, is specifically designed to calibrate the oscilloscope's time base and vertical deflection factor. For example, the COS5041 oscilloscope standard signal source 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 phosphor screen. Rotate the horizontal displacement knob (labeled with horizontal two-way arrow) to move the signal waveform left and right, and rotate the vertical displacement knob (labeled with vertical two-way 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 ways to select the input channel: 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 selecting DUAL, the oscilloscope displays the signal of channel 1 and channel 2 at the same time. When testing signals, first connect the ground of the oscilloscope to the ground of the circuit under test. According to the selection of the input channel, the oscilloscope probe is plugged into the corresponding channel socket, the ground on the oscilloscope probe is connected to the ground of the circuit under test, and the oscilloscope probe contacts the point under test. There is a two-position switch on the oscilloscope probe. This switch is dialed to the "× 1" position, the measured signal is sent to the oscilloscope without attenuation, and the voltage value read out from the fluorescent screen is the actual voltage value of the signal. This switch is dialed to "× 10" position, the measured signal attenuation for 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
There are three choices for input coupling: AC, GND and DC. When "Ground" is selected, the scan line shows the position of "Oscilloscope Ground" on the fluorescent screen. DC coupling is used to determine the absolute DC value of signals and to observe very low frequency signals. AC coupling is used to observe AC and AC signals with DC components. In digital circuit experiments, the general choice of "DC" mode, in order to observe the absolute voltage value of the signal. 2.5 Trigger
The first section pointed out that the measured signal from the Y-axis input, part of the Y-axis deflection plate sent to the oscilloscope, drive the point of light in the fluorescent screen proportional to the vertical direction of movement; the other part of the shunt to the x-axis deflection system to generate trigger pulses, triggering the scanning generator to generate a repetitive sawtooth wave voltage is added to the oscilloscope's X-deflector plate, so that the light point of the horizontal direction, the two together, the light point in the fluorescent screen, and the two together! The light point on the fluorescent screen depicts the graph of the signal under test. It can be seen that the correct trigger mode directly affects the effective operation of the oscilloscope. In order to get a stable, clear signal waveform on the fluorescent screen, it is very important to master the basic trigger function and its operation.
1. Trigger source (Source) selection
To enable the screen to display a stable waveform, it is necessary to be measured by the signal itself or with the measured signal has a certain time relationship between the trigger signal added to the trigger circuit. Trigger source selection determines where the trigger signal is supplied from. There are usually three trigger sources: internal trigger (INT), power trigger (LINE), external trigger EXT).
The internal trigger uses the signal under test as the trigger signal, which is a frequently used trigger method. Since the trigger signal itself is part of the signal under test, it can show a very stable waveform on the screen. Channel 1 or Channel 2 of the dual-trace oscilloscope can be selected as the trigger signal.
Power triggering uses the AC power frequency signal as the trigger signal. This method is effective in measuring signals related to the frequency of the AC power supply. It is especially effective when measuring audio circuits and low-level AC noise from gate tubes.
External triggering uses an external signal as the trigger signal, which is input from the external trigger input. The external trigger signal should have a periodic relationship with the signal under test. Since the signal under test is not used as a trigger signal, when the scanning starts is independent of the signal under test.
The correct selection of the trigger signal has a great deal to do with the stability and clarity of the waveform display. For example, in the measurement of digital circuits, for a simple periodic signal, the choice of internal triggering may be better, while for a signal with a complex cycle, and the existence of a signal with its periodic relationship, the choice of external triggering may be better.
2. Trigger coupling (Coupling) mode selection
Trigger signal to the trigger circuit of the coupling mode has a variety of ways, the purpose is to trigger the signal stability, reliability. Here are a few commonly used ones.
AC coupling is also known as capacitive coupling. It allows only the AC component of the trigger signal to be triggered, and the DC component of the trigger signal is isolated. This type of coupling 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 triggering difficulties.
Direct current (DC) coupling 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.
Low-frequency suppression (LFR) triggering trigger signal through the high-pass filter added to the trigger circuit, the trigger signal of the low-frequency component is suppressed; high-frequency suppression (HFR) triggering, the trigger signal through the low-pass filter added to the trigger circuit, the trigger signal of the high-frequency component is suppressed. In addition, there are also used for television maintenance of television synchronization (TV) trigger. These trigger coupling methods have their own scope of application, need to be in use to experience.
3. Trigger level (Level) and trigger polarity (Slope)
Trigger level adjustment is also called synchronization adjustment, which makes the scanning and the measured signal synchronization. The Level 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. By turning the knob clockwise, the trigger level rises; by turning the knob counterclockwise, the trigger level falls. 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, the level knob can not stabilize the trigger, with the release of the suppression (Hold Off) knob to adjust the release of the waveform suppression time (scanning pause time), can make the scanning and waveform stable synchronization.
The polarity switch is used to select the polarity of the trigger signal. Toggle in the "+" position, in the direction of the signal increase, when the trigger signal exceeds the trigger level, the trigger is generated. In the "-" position, the trigger is generated when the trigger signal exceeds the trigger level in the direction of decreasing signal. Trigger polarity and trigger level **** the same decision trigger signal trigger point. 2.6 Scanning mode (SweepMode)
Scanning has automatic (Auto), normal (Norm) and single (Single) three kinds of scanning mode.
Auto: When there is no trigger signal input, or the trigger signal frequency is lower than 50Hz, the scanning is self-excited.
Normal: When there is no trigger signal input, the scanning is in the ready state and there is no scanning line. When the trigger signal arrives, the scan is triggered.
Single: The single button is similar to a reset switch. In single scan mode, the scanning circuit is reset when the single button is pressed, and the Ready light is on at this time. The trigger signal arrives to generate a scan. When the single scan is finished, the Ready lamp goes out. A single scan is used to observe a non-periodic signal or a single transient signal, which often requires taking a picture of the waveform.
The above summarizes the basic functions and operations of an oscilloscope. Oscilloscopes also have some more complex functions, such as delayed scanning, trigger delay, X-Y mode of operation, etc., which will not be introduced here. It's easy to get started with oscilloscopes, but the real skill is in the application.