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Oscilloscope

An oscilloscope is a very widely used electronic measuring instrument. It can transform electrical signals that are invisible to the naked eye into visible images, which is convenient for people to study the changing process of various electrical phenomena. The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit a screen coated with a fluorescent substance to produce a small light spot (this is the working principle of a traditional analog oscilloscope). Under the effect of the measured signal, the electron beam is like the tip of a pen, and the curve of the instantaneous value of the measured signal can be drawn on the screen. Using an oscilloscope, you can observe the waveform curves of various signal amplitudes changing with time, and you can also use it to test various electrical quantities, such as voltage, current, frequency, phase difference, amplitude adjustment, etc.

Oscilloscope

Introduction

An oscilloscope is an instrument used to measure the shape of alternating current or pulsed current waves. It is composed of a tube amplifier, a scanning oscillator, and a cathode ray tube. In addition to observing the current waveform, you can also measure the frequency, voltage strength, and so on. Any periodic physical process that can become an electrical effect can be observed with an oscilloscope.

Classification

Classification according to different signals

The analog oscilloscope uses an analog circuit (oscilloscope, which is based on an electron gun). The electron gun emits electrons to the screen. The emitted electrons are focused to form an electron beam and hit the screen. The inner surface of the screen is coated with fluorescent material, so that the point hit by the electron beam will emit light.

Digital oscilloscopes are high-performance oscilloscopes manufactured by a series of technologies such as data acquisition, A/D conversion, and software programming. The working mode of the digital oscilloscope is to convert the measured voltage into digital information through an analog converter (ADC). The digital oscilloscope captures a series of samples of the waveform and stores the samples. The storage limit is to determine whether the accumulated samples can draw the waveform, and then the digital oscilloscope reconstructs the waveform. Digital oscilloscopes can be divided into digital storage oscilloscopes (DSO), digital phosphor oscilloscopes (DPO) and sampling oscilloscopes.

To increase the bandwidth of analog oscilloscopes, oscilloscopes, vertical magnification, and horizontal scanning must be fully promoted. To improve the bandwidth of a digital oscilloscope, only the performance of the front-end A/D converter needs to be improved, and there are no special requirements for the oscilloscope and scanning circuit. In addition, the digital oscilloscope can make full use of memory, storage and processing, as well as a variety of triggering and advanced triggering capabilities. In the 1980s, digital oscilloscopes sprung up, and the results were numerous. There was a tendency to completely replace analog oscilloscopes. Analog oscilloscopes did indeed retreat from the foreground to the background.

According to different structure and performance

①Ordinary oscilloscope. The circuit structure is simple, the frequency band is narrow, and the scanning linearity is poor. It is only used to observe the waveform.

②Multi-purpose oscilloscope. The frequency band is wide and the scanning linearity is good. It can quantitatively test DC, low frequency, high frequency, ultra high frequency signals and pulse signals. With the aid of amplitude calibrator and time calibrator, the measurement accuracy can reach ±5%.

③Multi-line oscilloscope. The multi-beam oscilloscope can simultaneously display the waveform of more than two signals of the same frequency on the fluorescent screen, without time difference, and the timing relationship is accurate.

④Multi-track oscilloscope. With the structure of electronic switch and gate control circuit, the waveform of more than two signals of the same frequency can be displayed simultaneously on the fluorescent screen of a single beam oscilloscope. However, there is a time difference and the timing relationship is not accurate.

⑤ Sampling oscilloscope. Sampling technology is used to convert high frequency signals into analog low frequency signals for display, and the effective frequency band can reach GHz level.

⑥Memory oscilloscope. Adopt storage oscilloscope or digital storage technology to keep single electrical signal transient process, aperiodic phenomenon and ultra-low frequency signal on the oscilloscope's fluorescent screen for a long time or store it in the circuit for repeated testing.

⑦Digital oscilloscope. There is a microprocessor inside and a digital display on the outside. Some products can display waveforms and characters on the oscilloscope fluorescent screen. The measured signal is sent to the data memory through the analog-to-digital converter (A/D converter), and can be added, subtracted, multiplied, divided, averaged, and square rooted to the captured waveform parameter data by keyboard operation , Find the root mean square value, etc., and display the answer number.

Basic composition

Display circuit

The display circuit includes the oscilloscope and its control circuit. The oscilloscope is a special kind of electron tube and an important part of an oscilloscope. The oscilloscope consists of three parts: electron gun, deflection system and fluorescent screen.

(1) Electronic gun

The electron gun is used to generate and form a high-speed, bunched electron flow to bombard the fluorescent screen to make it glow. It is mainly composed of filament F, cathode K, control pole G, first anode A1 and second anode A2. Except for the filament, the structure of the other electrodes is a metal cylinder, and their axes are kept on the same axis. After the cathode is heated, it can emit electrons in the axial direction; the control electrode has a negative potential relative to the cathode. Changing the potential can change the number of electrons passing through the control pinhole, that is, the brightness of the light spot on the fluorescent screen. In order to increase the brightness of the light spot on the screen without reducing the sensitivity to electron beam deflection, a modern accelerating tube also includes a rear acceleration electrode A3 between the deflection system and the fluorescent screen.

The first anode applies a positive voltage of about several hundred volts to the cathode. A higher positive voltage is applied to the second anode than the first anode. The electron beam passing through the extremely small control hole is accelerated by the high potential of the first anode and the second anode, and moves at a high speed in the direction of the phosphor screen. Due to the repulsion of the same charge, the electron beam will gradually spread out. Through the focusing effect of the electric field between the first anode and the second anode, the electrons are re-assembled and converge at one point. Properly controlling the size of the potential difference between the first anode and the second anode can make the focus just fall on the fluorescent screen, showing a bright and small dot. Changing the potential difference between the first anode and the second anode can play the role of adjusting the focus of the light spot. This is the principle of "focus" and "auxiliary focus" adjustment of the oscilloscope. The third anode is formed by coating a layer of graphite inside the cone of the oscilloscope, usually with a very high voltage, it has three functions: ① to further accelerate the electrons after passing through the deflection system, so that the electrons have enough energy to go Bouncing the phosphor screen to obtain sufficient brightness; ②The graphite layer is coated on the entire cone, which can play a shielding role; ③The electron beam bombarding the phosphor screen will generate secondary electrons, and A3 at a high potential can absorb these electrons.

(2) Deflection system

The deflection system of the oscilloscope is mostly an electrostatic deflection type, which is composed of two pairs of parallel parallel metal plates, which are called horizontal deflection plate and vertical deflection plate, respectively. Control the movement of the electron beam in the horizontal and vertical directions respectively. When the electrons move between the deflection plates, if there is no voltage applied to the deflection plates, there is no electric field between the deflection plates, and the electrons entering the deflection system after leaving the second anode will move in the axial direction and shoot toward the center of the screen. If there is voltage on the deflection plate and there is an electric field between the deflection plates, the electrons entering the deflection system will be directed to the designated position of the fluorescent screen under the action of the deflection electric field.

If the two deflection plates are parallel to each other and their potential difference is equal to zero, then the electron beam with velocity υ passing through the deflection plate space will move along the original direction (set as the axis direction) and hit the coordinate origin of the fluorescent screen . If there is a constant potential difference between the two deflection plates, an electric field is formed between the deflection plates. This electric field is perpendicular to the moving direction of the electrons, and the electrons are deflected toward the deflection plate with a higher potential. In this way, in the space between the two deflection plates, the electron moves tangentially at this point along the parabola. Finally, the electrons land on point A on the phosphor screen. This point A is a distance away from the origin (0) of the phosphor screen. This distance is called the deflection and is represented by y. The amount of deflection y is proportional to the voltage Vy applied to the deflection plate. Similarly, when a DC voltage is applied to the horizontal deflection plate, a similar situation occurs, except that the light spot is deflected in the horizontal direction.

(3) Fluorescent screen

The fluorescent screen is located at the terminal of the oscilloscope. Its function is to display the deflected electron beam for easy observation. A layer of luminescent substance is coated on the inner wall of the oscilloscope's phosphor screen. Therefore, the location on the phosphor screen exposed to high-speed electrons shows fluorescence. The brightness of the light spot at this time depends on the number, density and speed of the electron beam. When the voltage of the control electrode is changed, the number of electrons in the electron beam will change accordingly, and the brightness of the light spot will also change. When using an oscilloscope, it is not advisable to allow a very bright light spot to be fixed at a position on the oscilloscope fluorescent screen, otherwise the fluorescent substance at this point will be burned due to long-term impact of electrons, thus losing its luminous ability.

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Fluorescent screens coated with different fluorescent substances will show different colors and different afterglow time when impacted by electrons. Usually used to observe the general signal waveform is green light, which is a medium afterglow oscilloscope for observation of non-periodic The sex and low-frequency signals are used to emit orange-yellow light, which is a long afterglow oscilloscope; in the oscilloscope for photography, the blue afterglow oscilloscope is generally used.

Y-axis amplifier circuit

Since the deflection sensitivity of the oscilloscope is very low, for example, the commonly used oscilloscope 13SJ38J type, its vertical deflection sensitivity is 0.86mm/V (about 12V voltage produces 1cm of deflection), so the general signal voltage to be measured must first pass The amplification of the vertical amplifying circuit is added to the vertical deflection plate of the oscilloscope to obtain a figure of appropriate size in the vertical direction.

X-axis amplifier circuit

Since the deflection sensitivity of the oscilloscope in the horizontal direction is also very low, the voltage (sawtooth wave voltage or other voltage) connected to the horizontal deflection plate of the oscilloscope must also be amplified by the horizontal amplification circuit before being added to the oscilloscope’s Horizontally deflect the board to get a figure of appropriate size in the horizontal direction.

Scanning synchronization circuit

The scanning circuit generates a sawtooth wave voltage. The frequency of the sawtooth wave voltage can be continuously adjusted within a certain range. The role of the sawtooth voltage is to cause the electron beam emitted by the cathode of the oscilloscope to form a periodic horizontal displacement on the phosphor screen that is proportional to time, that is, to form a time baseline. In this way, the time-varying waveform of the measured signal applied in the vertical direction can be displayed on the fluorescent screen.

Power supply circuit

Power supply circuit: supply the negative high voltage and filament voltage required by the vertical and horizontal amplification circuits, scanning and synchronization circuits, oscilloscopes and control circuits.

It can be seen from the principle function of the oscilloscope that the measured signal voltage is applied to the Y-axis input end of the oscilloscope, and is added to the vertical deflection plate of the oscilloscope tube through the vertical amplification circuit. The horizontal deflection voltage of the oscilloscope, although the sawtooth voltage is used in most cases (when used to observe the waveform), sometimes other external voltages are used (when measuring frequency, phase difference, etc.), so at the input of the horizontal amplifier circuit There is a horizontal signal selection switch to select the sawtooth voltage inside the oscilloscope or other voltage applied to the X-axis input as the horizontal deflection voltage as required.

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In addition, in order to keep the graphics displayed on the fluorescent screen stable, it is required that the frequency of the sawtooth wave voltage signal and the frequency of the measured signal be kept synchronized. In this way, not only the frequency of the sawtooth wave voltage can be continuously adjusted, but also a synchronization signal must be input on the circuit that generates the sawtooth wave. In this way, for a simple oscilloscope (such as a domestic SB10 and other oscilloscopes) that can only generate one state of continuous scanning (that is, a continuous and continuous sawtooth wave), it is necessary to input a correlation with the frequency of the observed signal on its scanning circuit The synchronous signal to contain the oscillation frequency of the sawtooth wave. For the oscilloscope (such as domestic ST-16 type oscilloscope, SR-8 type double trace oscilloscope, etc.) with the function of waiting for scanning (that is, there is no sawtooth wave at ordinary times, a sawtooth wave is generated when the signal under test arrives, and a scan is performed) In order to adapt to various needs, the synchronization (or trigger) signal can be selected by the synchronization or trigger signal selection switch, usually there are 3 sources:

① The measured signal is drawn from the vertical amplifier circuit as a synchronization (or trigger) signal. This signal is called the "internal synchronization" (or "internal trigger") signal;

②Introduce some relevant externally added signal as a synchronization (or trigger) signal, this signal is called "external synchronization" (or "external trigger") signal, which is added to the external synchronization (or external trigger) input terminal;

③ The synchronization signal selection switch of some oscilloscopes also has a "power supply synchronization", which is a 220V, 50Hz power supply voltage, which is used as a synchronization signal after stepping down the transformer secondary voltage.

Fundamental

Waveform display

According to the principle of the oscilloscope, when a DC voltage is applied to a pair of deflection plates, the light spot will produce a fixed displacement on the fluorescent screen, and the magnitude of the displacement is proportional to the applied DC voltage. If two DC voltages are simultaneously applied to the two pairs of vertical and horizontal deflection plates, the position of the light spot on the fluorescent screen is determined by the displacement in both directions.

If a sinusoidal AC voltage is applied to a pair of deflection plates, the light spot will move with the change of voltage on the fluorescent screen. When a sinusoidal AC voltage is applied to the vertical deflection plate, the voltage is Vo (zero value) at the instant of time t=0, the position of the light spot on the fluorescent screen is at the coordinate origin 0, and at the instant of time t=1, the voltage is V1 (positive value), the light spot on the fluorescent screen is 1 above the coordinate origin 0, and the magnitude of the displacement is proportional to the voltage V1; at the instant of time t=2, the voltage is V2 (maximum positive value), the light spot on the fluorescent screen At 2 points above the origin of the coordinate 0, the displacement distance is proportional to the voltage V2; and so on, at each instant of time t=3, t=4,..., t=8, the positions of the light spots on the fluorescent screen are 3 , 4, ..., 8 o'clock. The second cycle, the third cycle of the AC voltage... will repeat the situation of the first cycle. If the frequency of the sinusoidal AC voltage applied to the vertical deflection plate at this time is very low, only 1 Hz to 2 Hz, then you will see a light spot moving up and down on the fluorescent screen. The instantaneous deflection value of this spot from the origin of the coordinate will be proportional to the instantaneous value of the voltage applied to the vertical deflection plate. If the frequency of the AC voltage applied to the vertical deflection plate is above 10 Hz to 20 Hz, due to the afterglow phenomenon of the fluorescent screen and the visual persistence of the human eye, what is seen on the fluorescent screen is not a point moving up and down, but a Vertical bright lines. The length of the bright line is determined by the peak-to-peak value of the sinusoidal AC voltage when the vertical amplification gain of the oscilloscope is constant. If a sinusoidal AC voltage is applied to the horizontal deflection plate, a similar situation occurs, except that the light spot moves on the horizontal axis.

If a voltage that changes linearly with time (such as a sawtooth wave voltage) is applied to a pair of deflection plates, how will the light spot move on the fluorescent screen? When there is a sawtooth wave voltage on the horizontal deflection plate, at time t=0, the voltage is Vo (maximum negative value), the starting position of the light spot on the left side of the coordinate origin on the fluorescent screen (at the zero point), the distance of displacement is proportional to Voltage Vo; at the instant of time t=1, the voltage is V1 (negative value), the spot on the screen is at a point to the left of the coordinate origin, and the displacement distance is proportional to the voltage V1; and so on, at time t=2 , T=3, ..., t=8, the corresponding positions of the light spots on the fluorescent screen are 2, 3, ..., 8 points. At the instant of t=8, the sawtooth wave voltage jumps from the maximum positive value V8 to the maximum negative value Vo, then the light spot on the fluorescent screen moves extremely quickly from 8 to the left to the starting position zero. If the sawtooth wave voltage is periodic, then the second cycle, the third cycle, ... will repeat the situation of the first cycle. If the frequency of the sawtooth wave voltage applied to the horizontal deflection plate at this time is very low, only 1Hz ~ 2Hz, on the fluorescent screen, you will see the light spot moving at a constant speed from the left zero position to 8 on the right, and then the light spot From 8 o'clock on the right, it moves extremely quickly to the zero on the left. This process is called scanning. When a periodic sawtooth wave voltage is applied to the horizontal axis, the scan will be repeated. The instantaneous value of the spot from the zero point of the starting position will be proportional to the instantaneous value of the voltage applied to the deflection plate. If the frequency of the sawtooth wave voltage applied to the deflection plate is above 10Hz to 20Hz, due to the afterglow phenomenon of the fluorescent screen and the persistence of the human eye, a horizontal bright line is seen. The length of the horizontal bright line is on the oscilloscope. The horizontal amplification gain is determined by the sawtooth wave voltage value. The sawtooth wave voltage value is proportional to the time change, and the displacement of the light spot on the fluorescent screen is proportional to the voltage value, so the horizontal bright line on the fluorescent screen can Represents the timeline. Any equal line segment on this bright line represents an equal period of time.

If the measured signal voltage is applied to the vertical deflection plate, the sawtooth wave scanning voltage is applied to the horizontal deflection plate, and the frequency of the measured signal voltage is equal to the frequency of the sawtooth wave scanning voltage, the fluorescent screen will display a period of measured Waveform curve of signal voltage with time. In the case where the second period, the third period, etc. of the periodic signal to be tested repeat the first period, the traces traced by the light spot on the fluorescent screen also overlap the traces traced for the first time. Therefore, the measured signal voltage displayed on the fluorescent screen is a stable waveform curve that changes with time.

In order to stabilize the graph on the fluorescent screen, the frequency of the measured signal voltage should maintain an integer ratio relationship with the frequency of the sawtooth wave voltage, that is, the synchronization relationship. In order to achieve this, the frequency of the sawtooth voltage needs to be continuously adjustable in order to adapt to the observation of periodic signals of various frequencies. Secondly, due to the relative instability of the frequency of the measured signal and the frequency of the sawtooth wave oscillation signal, even if the frequency of the sawtooth wave voltage is temporarily adjusted to an integral multiple relationship with the frequency of the measured signal, the graph cannot be kept stable all the time. Therefore, there are synchronization devices in the oscilloscope. That is, a synchronization signal is added to a part of the sawtooth wave circuit to promote the synchronization of the scan. For a simple oscilloscope (such as a domestic SB-10 type oscilloscope) that can only generate one state of continuous scanning (that is, a continuous sawtooth wave) Etc.), it is necessary to input a synchronization signal related to the frequency of the observed signal on its scanning circuit. When the frequency of the added synchronization signal is close to the autonomous oscillation frequency of the sawtooth wave frequency (or close to an integer multiple thereof), the sawtooth The wave frequency is "dragged into sync" or "locked". For oscilloscopes (such as domestic ST-16 oscilloscope, SBT-5 synchronous oscilloscope, SR-8) with the function of waiting for scanning (that is, there is no sawtooth wave at ordinary times, and a sawtooth wave is generated when the measured signal arrives for one scan) As for the dual-track oscilloscope, etc.), it is necessary to input a trigger signal related to the measured signal on its scanning circuit, so that the scanning process closely cooperates with the measured signal. In this way, as long as the appropriate synchronization signal or trigger signal is selected as required, any process to be studied can be synchronized with the sawtooth wave scanning frequency.

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Two-line oscilloscope

In the process of electronic practice technology, it is often necessary to observe the process of two (or more) signals changing with time at the same time. And test and compare the power of these different signals. In order to achieve this goal, on the basis of applying the principle of common oscilloscope, people adopt the following two methods of displaying multiple waveforms at the same time: one is the two-line (or multi-line) oscilloscope method; the other is the double trace (or Multitrack) Oscillographic method. Oscilloscopes manufactured using these two methods are called two-wire (or multi-line) oscilloscopes and two-track (or multi-track) oscilloscopes.

A two-wire (or multi-wire) oscilloscope is implemented with a two-gun (or multi-gun) oscilloscope. The following is a brief description using the double-gun oscilloscope as an example. The dual-gun oscilloscope has two independent electron guns to generate two electron beams. There are also two sets of independent deflection systems, each of which controls a beam of electrons to move up, down, left and right. The fluorescent screen is shared, so two different electrical signal waveforms can be displayed on the screen at the same time, and the two-wire oscilloscope can also be realized by using a single-gun two-wire oscilloscope. This type of oscilloscope has only one electron gun. During operation, it relies on special electrodes to split the electrons into two beams. Then, the two sets of independent deflection systems in the tube respectively control the movement of the two beams of electrons up and down, left and right. The fluorescent screen is shared and can display two different electrical signal waveforms at the same time. Due to the high manufacturing process requirements and high cost of the two-wire oscilloscope, the application is not very common.

Double trace oscilloscope

The dual-track (or multi-track) oscilloscope is based on a single-line oscilloscope, and a dedicated electronic switch is added to use it to display two (or more) waveforms separately. Because it is simpler to implement dual-track (or multi-track) oscilloscope than dual-line (or multi-line) oscilloscope, there is no need to use complicated and expensive “dual-cavity” or “multi-cavity” oscilloscopes, (Or multiple traces) oscilloscope has gained widespread application.

In order to keep the two signal waveforms displayed on the fluorescent screen stable, it is required that the measured signal frequency, the scanning signal frequency and the switching frequency of the electronic switch must satisfy a certain relationship.

First of all, the relationship between the frequency of the two measured signals and the frequency of the scanning signal should be an integer ratio, that is, "synchronization" is required. This point is the same as the principle of a single-line oscilloscope, the difference is that there are two signals to be measured, and the scanning voltage is one. In practical applications, the two signals that need to be observed and compared are often intrinsically related to each other, so the above synchronization requirements are generally easy to meet.

In order to stabilize the two measured signal waveforms displayed on the fluorescent screen, in addition to meeting the above requirements, the switching frequency of the electronic switch must also be reasonably selected so that the number of waveforms displayed on the oscilloscope is appropriate for easy observation. Let's talk about the working mode of electronic switches. This problem is related to the switching frequency of electronic switches. There are two ways of working for electronic switches: "alternating" conversion and "intermittent" conversion.

The waveform displayed by the alternating conversion working mode is very similar to the waveform displayed by the two-line oscilloscope method, and they have no discontinuities. However, since the waveforms of the measured signals UA and UB alternately appear on the fluorescent screen in sequence, if the alternate gap time exceeds the visual dwell time of the human eye and the afterglow time of the fluorescent screen, people will see on the fluorescent screen Will have flickering. In order to avoid this situation, it is required that the electronic switch has a sufficiently high switching frequency. This means that when the frequency of the signal under test is low, it is not advisable to use the alternate conversion working mode, but the intermittent conversion working mode. When the electronic switch uses the intermittent conversion working mode, in each process of the X-axis scanning, the electronic switch samples the measured signal for each display multiple times at a sufficiently high conversion frequency. In this way, even if the frequency of the measured signal is low, the flicker of the waveform can be avoided.

The dual trace oscilloscope is mainly composed of two channels of Y-axis pre-amplifier circuit, gate control circuit, electronic switch, hybrid circuit, delay circuit, Y-axis post-amplifier circuit, trigger circuit, scanning circuit, X-axis amplifier circuit, Z It consists of shaft amplifier circuit, calibration signal circuit, oscilloscope and high and low voltage power supply circuit.

When the display mode switch is placed in an alternating position, the electronic switch is a bistable circuit. It is controlled by the gate signal from the scanning circuit, so that the two front channels of the Y axis work alternately with the change of the scanning circuit gate signal. The number of alternate conversions per second is related to the repetition frequency of the scanning signal generated by the scanning circuit. The alternate working state is suitable for observing the measured signal with a frequency that is not too low.

In order to observe the waveform of the measured signal changing with time, a linear sweep voltage (sawtooth voltage) must be applied to the horizontal deflection plate of the oscilloscope. This scanning voltage is generated by the scanning circuit. When the trigger signal is added to the trigger circuit, the scan circuit is triggered, and the scan circuit generates the corresponding scan signal; when the trigger signal is not added, the scan circuit does not generate the scan signal.

There are two types of triggers: internal trigger and external trigger, which are selected by the trigger selection switch. When the switch is placed in the inner position, the trigger signal comes from the measured signal sent through the Y-axis channel. When the switch is placed in the external position, the trigger signal is sent from the outside. This signal should be in an integer ratio relationship with the frequency of the signal under test. Most of the oscilloscopes use the internal trigger working mode.

The low voltage in the high and low voltage power supply circuit is for the low voltage power supply required by the oscilloscope at all levels, and the high voltage is for the oscilloscope display system power supply.

Instrument classification

Oscilloscopes can be divided into analog oscilloscopes and digital oscilloscopes. For most electronic applications, both analog oscilloscopes and digital oscilloscopes are competent, but for some specific applications, due to the different characteristics of analog oscilloscopes and digital oscilloscopes, Appropriate and unsuitable places appear.

Analog

The working mode of an analog oscilloscope is to directly measure the signal voltage, and to draw the voltage in the vertical direction through the electron beam that passes through the oscilloscope screen from left to right.

Digital

The working mode of the digital oscilloscope is to convert the measured voltage into digital information through an analog converter (ADC). The digital oscilloscope captures a series of samples of the waveform and stores the samples. The storage limit is to determine whether the accumulated samples can draw the waveform, and then the digital oscilloscope reconstructs the waveform.

Digital oscilloscopes can be divided into digital storage oscilloscopes (DSO), digital phosphor oscilloscopes (DPO) and sampling oscilloscopes.

To increase the bandwidth of analog oscilloscopes, oscilloscopes, vertical magnification, and horizontal scanning must be fully promoted. To improve the bandwidth of a digital oscilloscope, only the performance of the front-end A/D converter needs to be improved, and there are no special requirements for the oscilloscope and scanning circuit. In addition, the digital oscilloscope can make full use of memory, storage and processing, as well as a variety of triggering and advanced triggering capabilities. In the 1980s, digital oscilloscopes sprung up, and the results were numerous. There was a tendency to completely replace analog oscilloscopes. Analog oscilloscopes did indeed retreat from the foreground to the background.

Parameter characteristics

Channel number classification

Whether it is an analog oscilloscope or a digital oscilloscope, it can be divided into: single channel/single trace oscilloscope; dual channel/dual trace oscilloscope; 2+1 channel (1 external trigger)/three trace oscilloscope; four channel/four trace Oscilloscope.

Bandwidth classification

The bandwidth is determined according to the test requirements of the oscilloscope, 5M/10M/20M/40M/60M/100M/1G...etc.

Instructions

Although oscilloscopes are divided into several categories, there are many models of various types, but the general oscilloscopes are not the same except for the bandwidth and input sensitivity. Take the SR-8 dual-track oscilloscope as an example.

(1) Analog oscilloscope panel device

The panel diagram of the SR-8 dual-track oscilloscope is shown above. The panel device can be divided into three parts according to its position and function: display, vertical (Y axis), horizontal (X axis). The functions of these three partial control devices are introduced separately.

1. The main control parts of the display part are:

(1) Power switch.

(2) Power indicator.

(3) Brightness Adjust the brightness of the light spot.

(4) Focus adjust the light spot or waveform clarity.

(5) Auxiliary focus Adjust the sharpness with the "focus" knob.

(6) Scale brightness Adjust the brightness of the scale line on the coordinate sheet.

(7) Tracking When the button is pressed down, the light spot deviating from the fluorescent screen is returned to the display area, and the light spot position is found.

(8) Standard signal output 1kHz, 1V square wave calibration signal is derived from this. Added to the Y-axis input to calibrate the Y-axis input sensitivity and X-axis scan speed.

2. Y-axis plug-in part

(1) The display mode selection switch is used to switch the control parts of the two Y-axis preamplifiers YA and YB. It has five different display modes:

"Alternate": When the display mode switch is set to "Alternate", the electronic switch is controlled and switched by the scanning signal, and the YA or YB signal is turned on in turn for each scan. When the frequency of the measured signal is higher, the frequency of the scanning signal is also higher. Electricity

The faster the sub-switch conversion rate, there will be no flicker. This working state is suitable for observing two signals with higher working frequency.

"Intermittent": When the display mode switch is set to "intermittent", the electronic switch is not controlled by the scanning signal, and a square wave signal with a fixed frequency of 200 kHz is generated, so that the electronic switch quickly turns on YA and YB alternately. Because the switching frequency is higher than the frequency of the measured signal, the signal waveforms of the two channels displayed on the screen are intermittent. When the frequency of the measured signal is high, the intermittent phenomenon is very obvious, and even cannot be observed; when the frequency of the measured signal is low, the intermittent phenomenon is masked. Therefore, this operating state is suitable for observing two signals with lower operating frequencies.

"YA", "YB": When the display mode switch is set to "YA" or "YB", it means that the oscilloscope is working in a single channel. At this time, the oscilloscope works like a single-track oscilloscope, that is, it can only display "YA" or The signal waveform of the "YB" channel.

"YA + YB": When the display mode switch is set to "YA + YB", the electronic switch does not work, the two signals of YA and YB pass through the amplifier and the gate circuit, and the oscilloscope will display the waveform of the two signals superimposed.

(2) "DC-⊥-AC" Y-axis input selection switch, used to select the coupling mode of the signal under test connected to the input terminal. Set to “DC” is direct coupling, can input AC signal containing DC component; set to “AC” position to realize AC coupling, only input AC component; when set to “⊥” position, Y-axis input terminal is grounded, which The time baseline displayed at the time is generally used as a reference baseline for testing the zero level of the DC voltage.

(3) "Fine adjustment V/div" sensitivity selection switch and fine adjustment device. The sensitivity selection is related to the shaft structure. The black knob is a Y-axis sensitivity coarse adjustment device, divided into 11 files from 10mv/div to 20v/div. The red knob is a fine adjustment device. When the clockwise direction is increased to full scale, it is the calibration position. You can read the amplitude of the measured signal according to the value indicated by the coarse adjustment knob. When the knob is turned counterclockwise to full scale, the change range should be greater than 2.5 times. Continuous adjustment of the "fine-tuning" potentiometer can achieve sensitivity coverage between the various levels. When making quantitative measurements, the knob should be placed in the The "calibration" position of the full scale of the hour hand.

(4) "Balance" When the input circuit of the Y-axis amplifier is unbalanced, the displayed light spot or waveform will be displaced along the Y-axis direction with the "fine adjustment" of the "V/div" switch. Adjust the "balance" potential The device can minimize this displacement.

(5) "↑↓" Y-axis displacement potentiometer is used to adjust the vertical position of the waveform.

(6) "Polarity, pull YA" The polarity of the YA channel is switched by a pull switch. When pulled out, the YA channel signal is displayed in reverse phase, that is, in the display mode (YA+YB), the display image is YB-YA.

(7) "Internal trigger, pull YB" trigger source selection switch. At the pressed position (normal state), the scanning trigger signal is taken from the input signals of the YA and YB channels respectively, which is suitable for single-track or double-track display, but it is not possible to compare the time of the double-track waveform. When the switch is pulled out, the trigger signal of the scan is only taken from the input signal of the YB channel, so it is suitable for comparing the time and phase difference of the two waveforms in the dual trace display.

(8) The Y-axis input socket adopts BNC type socket, and the signal under test is input directly or through the probe.

3. X axis plug-in part

(1) "t/div" scanning speed selection switch and fine adjustment knob. The movement speed of the X-axis light spot is determined by it, and it is divided into 21 levels from 0.2 μs to 1 s. When the switch "fine-adjust" the potentiometer to the clockwise direction and connect the switch, it is the "calibration" position. At this time, the indicated value of "t/div" is the actual value of the scanning speed.

(2) "Expand, pull × 10" scanning speed expansion device. It is a push-type switch, which is used normally in the pressed state, and the scanning speed of the pulled position is increased by 10 times. The indication value of "t/div" should also be calculated accordingly. Using "Extended Pull × 10" is suitable for observing waveform details.

(3) "→←" X-axis position adjustment knob. It is the horizontal axis adjustment potentiometer of the X-axis light trace, which is a sleeve shaft structure. The knob on the outer ring is a coarse adjustment device. Rotate the base line clockwise to move to the right, and turn it counterclockwise to move the base line to the left. The small knob placed on the sleeve shaft is a fine adjustment device, which is suitable for signal adjustment after expansion.

(4) "External trigger, X external" socket uses BNC type socket. When the external trigger is used, it is used as the socket for connecting the external trigger signal. It can also be used as a signal input socket when the X-axis amplifier is externally connected. Its input impedance is about 1MΩ. When used externally, the peak value of the input signal should be less than 12V.

(5) "Trigger level" knob Trigger level adjustment potentiometer knob. Used to select the trigger point of the input signal waveform. Specifically, it is to adjust the time to start the scan and determine at which point of the trigger signal waveform the scan is triggered. When rotating clockwise, the trigger point tends to the positive part of the signal waveform, and when rotating counterclockwise, the trigger point tends to the negative part of the signal waveform.

(6) "Stability" triggers the stability fine-tuning knob. Used to change the working state of the scanning circuit, it should generally be in a state to be triggered. The adjustment method is to put the Y-axis input coupling mode selection (AC-ground-DC) switch to the ground position, and the V/div switch to the highest sensitivity level. When the level knob is adjusted away from the self-excited state, use Use a small screwdriver to turn the stability potentiometer clockwise to the end, and the scanning circuit generates a self-excited scan. At this time, the scanning line appears on the screen; then slowly rotate counterclockwise to make the scanning line disappear. At this time, the scanning circuit is in a state to be triggered. In this state, when measuring with an oscilloscope, as long as the level knob is adjusted, a stable waveform can be obtained on the screen, and the starting point of the waveform on the screen can be adjusted at will. For a few oscilloscopes, when the stability potentiometer is turned counterclockwise to the end, a scan line appears on the screen; then slowly rotate it clockwise to make the scan line disappear on the screen, and the scan circuit is in a state to be triggered.

(7) "Internal and external" trigger source selection switch. When placed in the "inner" position, the scan trigger signal is taken from the measured signal of the Y-axis channel; when placed in the "outer" position, the trigger signal is taken from the external trigger signal introduced at the "external trigger X external" input terminal.

(8) "AC" "AC(H)" and "DC" trigger the coupling mode switch. The "DC" file is a DC coupling state, suitable for a trigger signal that changes slowly or has a very low frequency (such as less than 100Hz). The "AC" file is an AC coupling state. Since the DC component in the trigger is blocked, the trigger performance is not affected by the DC component. The "AC (H)" file is the AC coupling state with low frequency suppression. When observing the high-frequency composite wave containing low-frequency components, the trigger signal is coupled through the high-pass filter, which suppresses the low-frequency noise and low-frequency trigger signal (low frequency below 2MHz) Component), to avoid waveform sway caused by false triggering.

(9) "High frequency, normal, automatic" trigger mode switch. It is used to select different triggering methods to adapt to different tested signals and test purposes. "High frequency" file, select this file when the frequency is very high (such as higher than 5MHz), and there is not enough amplitude to stabilize the trigger. At this time, the scan is in the high-frequency trigger state, and the high-frequency signal (200 kHz signal) generated by the oscilloscope itself synchronizes the measured signal. It is not necessary to adjust the level knob frequently, the stable waveform can be displayed on the screen, and the operation is convenient, which is conducive to observing the high-frequency signal waveform. The "normal" file uses input signals from the Y axis or an external trigger source for trigger scanning, which is a commonly used trigger scanning method. "Auto" block, the scan is in the automatic state (similar to the high-frequency trigger mode), but you can observe the stable waveform without adjusting the level knob. It is easy to operate and is conducive to the observation of lower frequency signals.

(10) "+, -" trigger the polarity switch. Select the rising part of the trigger signal at the "+" position, and select the falling part of the trigger signal at the "-" position to trigger the scanning circuit.

(2) Inspection before use

Before the oscilloscope is used for the first time or when it is multiplexed for a long time, it is necessary to perform a simple check of whether it can work and to adjust the stability of the scanning circuit and the DC balance of the vertical amplification circuit. The oscilloscope must also calibrate the gain of the vertical amplifier circuit and the horizontal scanning speed when performing the quantitative test of voltage and time. The inspection method of the oscilloscope's normal operation, the calibration method of the vertical amplification circuit gain and the horizontal scanning speed, due to the different amplitude and frequency of the calibration signal of various types of oscilloscopes, the inspection and calibration methods are slightly different.

(3) Use steps

An oscilloscope can be used to observe the waveform curves of various electrical signal amplitudes changing with time. On this basis, the oscilloscope can be used to measure electrical parameters such as voltage, time, frequency, phase difference, and amplitude adjustment. The following describes the procedure for using an oscilloscope to observe the waveform of an electrical signal.

1. Select Y-axis coupling

According to the frequency of the measured signal frequency, the Y-axis input coupling mode selection "AC-Ground-DC" switch is placed in AC or DC.

2. Select Y axis sensitivity

According to the approximate peak-to-peak value of the measured signal (if an attenuation probe is used, it should be divided by the attenuation multiple; when the DC mode is used for the coupling mode, the superimposed DC voltage value must also be considered), select the V/div switch for the Y-axis sensitivity (or Y-axis attenuation switch) placed in the appropriate gear. If you don't need to read the measured voltage value in actual use, you can adjust the Y-axis sensitivity fine-tuning (or Y-axis gain) knob appropriately to make the desired height waveform appear on the screen.

3. Select trigger (or sync) signal source and polarity

Normally, the trigger (or synchronization) signal polarity switch is set to "+" or "-" position.

4. Select scan speed

According to the approximate value of the measured signal period (or frequency), the X-axis scanning speed t/div (or scanning range) switch is set to the appropriate level. If you do not need to read the time value in actual use, you can adjust the t/div fine-tuning (or scan fine-tuning) knob appropriately, so that the waveform of the number of cycles required for the test is displayed on the screen. If you need to observe the edge of the signal, the sweep t/div switch should be set to the fastest sweep speed.

5. Input signal under test

After the measured signal is attenuated by the probe (or directly input by the coaxial cable without attenuation, but the input impedance is reduced and the input capacitance is increased at this time), it is input to the oscilloscope through the Y-axis input terminal.

Common fault phenomena and causes

No light spots or waves

The power is not turned on.

The brightness knob is not adjusted well.

X, Y axis shift knob position adjustment.

Improper adjustment of the Y-axis balance potentiometer caused serious imbalance in the DC amplifier circuit.

Can't open horizontally

If the trigger source selection switch is set to the external gear and there is no external trigger signal input, no sawtooth wave is generated.

Improper adjustment of the level knob.

The stability potentiometer is not adjusted so that the scanning circuit is in a critical state to be triggered.

The X axis selection is mistakenly placed in the X external position, and there is no signal input on the external socket.

If the two-track oscilloscope only uses the A channel (the B channel has no input signal), and the internal trigger switch is set to the YB position, no sawtooth wave is generated.

No display in vertical direction

Input coupling mode DC-Ground-AC switch was mistakenly placed in the ground position.

The high and low potential terminals of the input terminal are reversely connected to the high and low potential terminals of the circuit under test.

The input signal is small, and the V/div is mistakenly placed in the low sensitivity range.

Unstable waveform

The stability potentiometer rotates excessively clockwise, causing the scanning circuit to be in a self-excited scanning state (not in a critical state to be triggered).

Trigger coupling mode AC, AC (H), DC switch failed to correctly select the corresponding gear level according to different trigger signal frequencies.

When the high-frequency trigger state is selected, the trigger source selection switch is mistakenly placed in the external gear (should be placed in the internal gear.)

When part of the oscilloscope scan is in automatic mode (continuous scan), the waveform is unstable.

Dense vertical lines or a rectangle

Improper selection of t/div switch causes f-scan <<f signal.

Dense horizontal lines or an oblique horizontal line

Improper selection of t/div off causes f-scan>>f signal.

The voltage reading in the vertical direction is not accurate

No vertical deflection sensitivity (v/div) calibration.

When performing v/div calibration, the v/div fine adjustment knob is not set to the correction position (that is, it is not turned fully clockwise).

During the test, the v/div fine adjustment knob was adjusted away from the correction position (that is, adjusted away from the position where the foot was turned clockwise).

A 10:1 attenuation probe is used, which is not multiplied by 10 times when calculating the voltage.

The frequency of the signal under test exceeds the maximum operating frequency of the oscilloscope, and the oscilloscope reading is smaller than the actual value.

The measured value is the peak-to-peak value, and the sine effective value needs to be calculated by conversion.

The horizontal reading is not accurate

No horizontal deflection sensitivity (t/div) calibration.

When performing t/div calibration, the t/div fine-tuning knob is not placed in the calibration position (that is, it is not turned fully clockwise).

During the test, the t/div fine adjustment knob was adjusted away from the correction position (that is, adjusted away from the position where the foot was turned clockwise).

When the sweep speed extension switch is placed in the pulled (×10) position, the test does not increase the sensitivity by 10 times according to the value indicated by the t/div switch.

The DC voltage value of the AC-DC superimposed signal is unclear

The Y-axis input coupling selection DC-Ground-AC switch is mistakenly placed in the AC range (should be placed in the DC range).

Before the test, the DC-ground-AC switch was not placed in the ground gear for DC level reference point correction.

The Y-axis balance potentiometer is not adjusted.

The phase difference between the two signals cannot be measured

The phase difference between the two signals cannot be measured (waveform display method)

The dual trace oscilloscope mistakenly puts the internal trigger (pull YB) switch to the pressed (normal) position and should put the switch to the YB position.

The dual trace oscilloscope did not correctly select the alternating and intermittent files of the display mode switch.

Single-line oscilloscope trigger selection switch is mistakenly placed in the internal gear.

Although the trigger selection switch of the single-line oscilloscope is placed in the external range, the two external triggers do not use the same signal.

Amplitude modulation waveform

The t/div switch is not properly selected, and the scan frequency is selected by the carrier wave frequency of the AM wave (it should be selected by the frequency of the audio AM signal).

The waveform cannot be adjusted to the required start time and position

The stability potentiometer is not adjusted at the critical trigger point to be triggered.

Trigger polarity (+, -) and trigger level (+, -) are not coordinated properly.

The trigger mode switch is mistakenly placed in the automatic gear (should be placed in the normal gear).

Triggered or synchronized scan

Slowly adjust the trigger level (or synchronization) knob to display a stable waveform on the screen. Adjust the level knob appropriately according to the observation needs to display the waveform at the corresponding starting position.

If you use a dual-track oscilloscope to observe the waveform and make a single-track display, the display mode switch is set to YA or YB. The signal under test is input to the oscilloscope through the YA or YB input terminal. The Y-axis trigger source selects the "internal trigger-pull YB" switch to be in the (normal) position. If the oscilloscope is displaying in two traces, the display mode switch is set to alternating gear (for observation of signals that are not too low in frequency), or intermittent gear (for observation of signals that are not too high in frequency), and the Y-axis trigger source Select the "internal trigger-pull YB" switch to set the "pull YB" file.

Abnormalities caused by improper use

During the use of the oscilloscope, the operator often does not understand the principle of oscilloscope and is unfamiliar with the function of the oscilloscope panel control device, which may cause abnormal phenomena due to improper adjustment.

Test application

Voltage measurement

Any measurement made with an oscilloscope comes down to the measurement of voltage. The oscilloscope can measure the voltage amplitude of various waveforms, not only DC voltage and sinusoidal voltage, but also pulse or non-sinusoidal voltage. More useful is that it can measure the voltage amplitude of each part of a pulse voltage waveform, such as the amount of overshoot or the amount of top drop. This is unmatched by any other voltage measuring instrument.

1. Direct measurement

The so-called direct measurement method is to measure the height of the measured voltage waveform directly from the screen, and then convert it into a voltage value. When measuring the voltage quantitatively, generally turn the fine-tuning knob of the Y-axis sensitivity switch to the "calibration" position, so that the measured value of the vertical axis taken by the measured value of the "V/div" and the measured signal can be directly calculated. Voltage value. Therefore, the direct measurement method is also called the ruler method.

(1) Measurement of AC voltage

Place the Y-axis input coupling switch in the "AC" position to display the AC component of the input waveform. If the frequency of the AC signal is very low, the Y-axis input coupling switch should be placed in the "DC" position.

Move the measured waveform to the center of the oscilloscope screen, use the "V/div" switch to control the measured waveform within the effective working area of the screen, and read the entire waveform occupied by the Y axis according to the division of the coordinate scale. In the direction of degree H, the peak-to-peak value VP-P of the measured voltage can be equal to the product of the "V/div" switch indication value and H. If the probe is used for measurement, the attenuation of the probe should be calculated, that is, multiply the calculated value by 10.

For example, the Y-axis sensitivity switch "V/div" of the oscilloscope is in the 0.2 level, and the measured amplitude of the coordinate H of the Y-axis is 5 div, then the peak-to-peak value of this signal voltage is 1V. If it is measured by the probe and still indicates the above value, the peak-to-peak value of the measured signal voltage is 10V.

(2) Measurement of DC voltage

Set the Y-axis input coupling switch to the "ground" position, and the trigger mode switch to the "automatic" position, so that the screen displays a horizontal scanning line, which is a zero-level line.

Set the Y-axis input coupling switch to the "DC" position and add the measured voltage. At this time, the scan line generates a jump displacement H in the Y-axis direction. The measured voltage is the product of the "V/div" switch indication value and H.

The direct measurement method is simple and easy, but the error is large. The factors that cause errors are reading errors, parallax and oscilloscope system errors (attenuator, deflection system, oscilloscope edge effect), etc.

2. Comparative measurement

The comparative measurement method is to obtain the measured voltage value by comparing a known standard voltage waveform with the measured voltage waveform.

Input the measured voltage Vx into the Y-axis channel of the oscilloscope, adjust the Y-axis sensitivity selection switch "V/div" and its fine adjustment knob, so that the fluorescent screen displays the height Hx that is convenient for measurement and make a record, and the "V/div" switch and The position of the fine adjustment knob remains unchanged. Remove the measured voltage, input a known adjustable standard voltage Vs to the Y axis, adjust the output amplitude of the standard voltage, so that it displays the same amplitude as the measured voltage. At this time, the output amplitude of the standard voltage is equal to the amplitude of the measured voltage. The voltage measurement by the comparison method can avoid the errors and errors caused by the vertical system, thus improving the measurement accuracy.

Time measurement

The oscilloscope time base can produce scan lines that are linearly related to time, so the horizontal scale of the fluorescent screen can be used to measure the time parameters of the waveform, such as the repetition period of the periodic signal, the width of the pulse signal, the time interval, the rise time (leading edge) and Fall time (back porch), time difference between the two signals, etc.

When the "fine adjustment" device of the oscilloscope's sweep switch "t/div" is turned to the calibration position, the time represented by the horizontal scale of the displayed waveform can be directly read and calculated according to the indication value of the "t/div" switch. Calculate the time parameter of the measured signal accurately.

Phase measurement

Using an oscilloscope to measure the phase difference between two sinusoidal voltages has practical significance. A counter can measure frequency and time, but it cannot directly measure the phase relationship between sinusoidal voltages. There are many ways to measure the phase using an oscilloscope. Below, only a few common methods are introduced.

1. Double trace method

The dual-track method uses a dual-track oscilloscope to directly compare the waveforms of the two measured voltages on the phosphor screen to measure its phase relationship. During the measurement, the signal with advanced phase is connected to the YB channel, and the other signal is connected to the YA channel. Use YB trigger. Adjust the "t/div" switch so that one cycle of the measured waveform accurately occupies 8 div on the horizontal scale. In this way, the phase angle of 360° of one cycle is divided into 8 equal parts, and every 1 div is equivalent to 45°. Read the difference T between the leading wave and the lagging wave on the horizontal axis, and calculate the phase difference φ as follows:

φ=45°/div×T (div)

If T==1.5div, then φ=45°/div×1.5div=67.5°

2. Graphical phase measurement

Put the X-axis of the oscilloscope at the X-axis input position, connect the signal u1 to the Y-axis input of the oscilloscope, and the signal u2 to the X-axis input of the oscilloscope. Properly adjust the relevant knob on the oscilloscope panel to make an ellipse of appropriate size appear on the fluorescent screen (in a special case, it may be a perfect circle or a diagonal line).

Suppose the signal u1 on the Y-axis deflection plate leads the signal u21/8 cycle on the X-axis deflection plate, and set the initial phase of u2 to zero, that is, φ2=0, so when u2 is zero, u1 is a larger value. As shown in the "0" point. At this time, the light spot on the fluorescent screen is correspondingly located at the "0" point. As time changes, u1 rises and u2 also rises, and the spot on the screen moves to the upper right. After 1/8 cycle, u1 and u2 respectively reach the "1" point, at this time u1 reaches the maximum value, u2 is a larger value, and the light spot on the fluorescent screen is located at the corresponding "1". If you continue this way, the light spot on the screen will trace an ellipse rotating clockwise. If u1 lags behind u2, a counterclockwise ellipse is formed. Of course, this is only when the signal frequency is very low (such as a few hertz), and the phenomenon of the light spot on the phosphor screen rotating clockwise or counterclockwise can be clearly seen on the phosphor screen with short afterglow. It can be seen from the above that the shape of the ellipse varies with the phase difference between the two sinusoidal signal voltages u1 and u2. Therefore, the phase difference Δφ between the two sinusoidal signals can be determined according to the shape of the ellipse. Let A be the ordinate of the intersection of the ellipse and the Y axis, and B be the maximum coordinate of each point on the ellipse. It can be seen from the figure that A is the instantaneous voltage corresponding to u1 at t=0, namely

A=Um1sinφ1

B is the amplitude corresponding to u1, ie

B=Um1

So A/B=(Um1sinφ1)/ Um1= sinφ1

To represent. In the actual test, it is convenient to read, often read 2A, 2B (or 2C, 2D), according to the formula

Δφ=arc sin (2A/2B) or Δφ=arc sin (2C/2D) to calculate the phase difference.

If the main axis of the ellipse is in the first and third quadrants, the phase difference is between 0° to 90° or 270° to 360°; if the main axis is in the second and fourth quadrants, the phase difference is 90° to 180° Or between 180°~270°.

Frequency measurement

There are many ways to measure the signal frequency with an oscilloscope. Here are two basic methods that are commonly used.

1. Periodic method

For any periodic signal, the aforementioned time interval measurement method can be used to first determine the time T of each cycle, and then use the following formula to find the frequency f: f=1/T

For example, the measured waveform displayed on the oscilloscope has a period of 8 div. The “t/div” switch is set to the “1 μs” position, and its “fine adjustment” is set to the “calibration” position. Then its period and frequency are calculated as follows:

T=1us/div×8div = 8us

f = 1/8us =125kHz

Therefore, the frequency of the measured waveform is 125kHz.

2. Graphic method to measure frequency

Set the oscilloscope to the X-Y working mode, input the measured signal to the Y axis, input the standard frequency signal to "X external", and slowly change the standard frequency to make the two signal frequencies become an integer multiple, such as fx:

fy=1:2, a stable pattern will be formed on the fluorescent screen.

The shape of the graph is not only related to the phase of the two deflection voltages, but also to the frequency of the two deflection voltages. The tracing method can be used to draw the graphs of various frequency ratios and different phase differences between ux and uy.

Using the relationship between the graph and the frequency, an accurate frequency comparison can be made to determine the frequency of the measured signal. The method is to draw the horizontal line and the vertical line through the graphic respectively. The horizontal line and vertical line drawn should not pass through the intersection of the graphic or be tangent to it. If the number of intersections between the horizontal line and the graph is m, and the number of intersections between the vertical line and the graph is n, then

fy / fx=m / n

When the standard frequency fx (or fy) is known, the measured signal frequency fy (or fx) can be obtained from the above formula. Obviously, in the actual test work, when using Lissajous graphics for frequency testing, in order to make the test simple and correct, if conditions permit, usually adjust the frequency of the known frequency signal as much as possible, so that the graph displayed on the fluorescent screen is a circle or ellipse . At this time, the measured signal frequency is equal to the known signal frequency.

Due to the different phases of the two voltages applied to the oscilloscope, the graphics on the fluorescent screen will have different shapes, but this has no effect on determining the unknown frequency.

Other related

Precautions

The safety of the instrument operator and the safety of the instrument. The instrument works normally within the safe range to ensure accurate measurement waveforms and reliable data. Attention should be paid to:

1. The general oscilloscope adjusts the brightness and focus knob to minimize the diameter of the light spot to make the waveform clear and reduce the test error; do not leave the light spot at a standstill, otherwise the electron beam bombarding should form a dark spot on the fluorescent screen and damage the fluorescent screen.

2. Measurement systems-such as oscilloscopes, signal sources; printers, computers, etc. The grounding wire of the electronic equipment under test-such as instruments, electronic components, circuit boards, and power supply of the equipment under test must be connected to the common ground (earth).

3. When the TDS200/TDS1000/TDS2000 series digital oscilloscope is used with a probe, it can only measure the waveform of the signal (the measured signal-the signal ground is the ground, and the output amplitude of the signal terminal is less than 300V CAT II). Never measure the AC220V or floating signal of electronic equipment that cannot be isolated from AC220V. (The floating floor can not be connected to the earth, otherwise it will cause damage to the instrument, such as testing the electromagnetic oven.)

4. The shell of the general oscilloscope, the metal outer ring of the signal input terminal BNC socket, the ground wire of the probe, and the ground wire end of the AC220V power socket are all connected. If the instrument is not connected to the ground wire and the probe is used to measure the floating signal directly, the instrument will generate a potential difference between the ground; the voltage value is equal to the potential difference between the point where the probe ground wire contacts the device under test and the ground. This will bring serious safety hazards to instrument operators, oscilloscopes, and electronic equipment under test.

5. If the user needs to measure switching power supply (switching power supply primary, control circuit), UPS (uninterruptible power supply), electronic rectifier, energy-saving lamp, inverter and other types of products or other electronic equipment that is not isolated from the mains AC220V for floating signal testing , Must use DP100 high voltage isolated differential probe.

Other matters needing attention in the use of oscilloscope:

(1) Thermal electronic instruments generally avoid frequent power-on and power-off, as do oscilloscopes.

(2) If you find that the waveform is disturbed by the outside world, you can ground the oscilloscope case.

(3) The voltage of "Y input" should not be too high, so as not to damage the instrument, and it should not exceed 400 V at the maximum attenuation. When the "Y input" wire is suspended, interference waveforms appear due to external electromagnetic interference, and this phenomenon should be avoided.

(4) Turn off the brightness adjustment knob in the counterclockwise direction before turning off the machine to minimize the brightness, and then turn off the power switch.

(5) When observing and adjusting the bright spots on the screen, the brightness of the bright spots should be moderate and not too bright.

Oscilloscopes are divided into multi-purpose oscilloscopes, digital oscilloscopes, analog oscilloscopes, virtual oscilloscopes, arbitrary waveform oscilloscopes, handheld oscilloscopes, digital fluorescence oscilloscopes, and data acquisition oscilloscopes.

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