The oscilloscope is a very versatile electronic measuring instrument. It can convert 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 the phosphor-coated screen to produce a small light spot (this is the working principle of a traditional analog oscilloscope).
Under the action of the measured signal, the electron beam is like the tip of a pen, which can draw the change curve of the instantaneous value of the measured signal on the screen. The oscilloscope can be used to observe the waveform curves of various signal amplitudes changing with time, and it can also be used to test various electric quantities, such as voltage, current, frequency, phase difference, amplitude modulation, and so on.
What is Oscilloscope?
An oscilloscope is an instrument used to measure the shape of alternating current or pulse current waves. It is composed of a tube amplifier, a scanning oscillator, a cathode ray tube, and so on. In addition to observing the waveform of the current, it can also measure the frequency, voltage intensity, etc. Any periodic physical process that can become an electrical effect can be observed with an oscilloscope.
Classification
According to the different classification of the signal
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 spot 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 model of a digital oscilloscope is to convert the measured voltage into digital information through an analog converter (ADC).
The digital oscilloscope captures a series of sample values of the waveform and stores the sample values. The storage limit is to judge 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 an analog oscilloscope, oscilloscope, vertical amplification, and horizontal scanning are required to fully advance. 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.
According to the structure and performance of different classification
①Ordinary oscilloscope
The circuit structure is simple, the frequency band is narrow, the scanning linearity is poor, and it is only used for observing the waveform.
②Multi-purpose oscilloscope
The frequency band is wide, the scanning linearity is good, and 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 use of multiple oscilloscope tubes can display the waveforms of more than two signals with the same frequency on the fluorescent screen at the same time, without time difference, and the timing relationship is accurate.
④Multi-track oscilloscope
With the structure of the electronic switch and gate control circuit, it can display the waveform of more than two signals with the same frequency on the phosphor screen of a single beam oscilloscope. However, there is a time difference and the timing relationship is not accurate.
⑤ Sampling oscilloscope
Using sampling technology to convert high-frequency signals into analog low-frequency signals for display, the effective frequency band can reach the GHz level.
⑥Memory oscilloscope
Using a storage oscilloscope or digital storage technology, single electrical signal transients, non-periodic phenomena, and ultra-low frequency signals are kept on the fluorescent screen of the oscilloscope or stored in the circuit for a long time for repeated testing.
⑦Digital oscilloscope
There is a microprocessor inside and a digital display outside. Some products can display waveforms and characters on the oscilloscope fluorescent screen.
The measured signal is sent to the data memory via the analog-to-digital converter (A/D converter). Through keyboard operation, the captured waveform parameter data can be added, subtracted, multiplied, divided, averaged, and square root calculated. , Calculate the root mean square value, etc., and display the answer number.
Basic composition
Display circuit
The display circuit includes an oscilloscope and its control circuit. The oscilloscope tube is a special kind of electronic tube and an important part of the oscilloscope. The oscilloscope tube is composed of three parts, the electron gun, a deflection system, and a phosphor screen.
(1) Electron gun
The electron gun is used to generate and form a high-speed, focused electron stream to bombard the phosphor screen to make it glow. It is mainly composed of filament F, cathode K, control electrode G, first anode A1, and second anode A2.
Except for the filament, the structures of the other electrodes are all metal cylinders, 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 hole, that is, control the brightness of the light spot on the phosphor screen.
In order to increase the brightness of the light spot on the screen without reducing the sensitivity to electron beam deflection, in modern oscilloscopes, a back acceleration electrode A3 is added between the deflection system and the phosphor screen.
A positive voltage of about several hundred volts is applied from the first anode to the cathode. A higher positive voltage is applied to the second anode than the first anode. The electron beam passing through the 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.
As the charges of the same sex repel each other, 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 regroup and converge at one point. Properly controlling the magnitude of the potential difference between the first anode and the second anode can make the focus just fall on the phosphor screen, showing a bright small dot.
Changing the potential difference between the first anode and the second anode can adjust the focus of the light spot. This is the principle of the 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. It is usually applied with a very high voltage. It has three functions.
①Further accelerate the electrons after passing through the deflection system, so that the electrons have enough energy to bombard 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 bombards the phosphor screen to generate secondary electrons, and A3 at high potential can absorb these electrons.
(2) Deflection system
The deflection system of the oscilloscope is mostly an electrostatic deflection type, which consists of two pairs of parallel metal plates that are perpendicular to each other, called the horizontal deflection plate and the vertical deflection plate. Control the movement of the electron beam in the horizontal and vertical directions respectively.
When electrons move between the deflection plates, if there is no voltage applied to the deflection plates and there is no electric field between the deflection plates, the electrons entering the deflection system after leaving the second anode will move along the axial direction and shoot toward the center of the screen. If there is a voltage on the deflection plate, there is an electric field between the deflection plates, and the electrons entering the deflection system will be directed to the designated position of the phosphor screen under the action of the deflection electric field.
(3) Fluorescent screen
The fluorescent screen is located at the terminal of the oscilloscope, and its function is to display the deflected electron beam for easy observation. The inner wall of the phosphor screen of the oscilloscope is coated with a layer of luminescent material so that the place on the phosphor screen that is impacted by high-speed electrons will show fluorescence. At this time, the brightness of the light spot depends on the number, density, and speed of the electron beams.
Y-axis amplifier circuit
Because the deflection sensitivity of the oscilloscope tube is very low, such as the commonly used oscilloscope tube 13SJ38J, its vertical deflection sensitivity is 0.86mm/V (about 12V voltage produces 1cm deflection), so the general measured signal voltage must pass through The magnification of the vertical amplifying circuit is added to the vertical deflection plate of the oscilloscope to obtain a pattern 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 voltage or other voltage) connected to the horizontal deflection plate of the oscilloscope must be amplified by the horizontal amplifier circuit before being added to the oscilloscope. Deflection the board horizontally to get a pattern of appropriate size in the horizontal direction.
Scan synchronization circuit
The scanning circuit generates a sawtooth wave voltage. The frequency of the sawtooth voltage can be continuously adjusted within a certain range. The function of the sawtooth wave voltage is to make the electron beam emitted from the cathode of the oscilloscope form a periodic horizontal displacement in direct proportion to the time on the phosphor screen, that is, to form a time baseline. In this way, the measured signal added in the vertical direction can be displayed on the fluorescent screen according to the time-varying waveform.
Power supply circuit
Power supply circuit supplies the negative high voltage and filament voltage required by the vertical and horizontal amplifying circuit, scanning and synchronization circuit, oscilloscope, and control circuit.
It can be seen from the principal function of the oscilloscope that the measured signal voltage is added to the Y-axis input end of the oscilloscope and added to the vertical deflection plate of the oscilloscope through the vertical amplifying circuit. Although the horizontal deflection voltage of the oscilloscope tube uses sawtooth voltage in most cases, other external voltages are sometimes used.
Therefore, there is a horizontal signal selection switch at the input of the horizontal amplifying circuit to select the sawtooth voltage inside the oscilloscope as needed. Or select other voltages applied to the X-axis input terminal as the horizontal deflection voltage.
In addition, in order to keep the graphics displayed on the phosphor screen stable, the frequency of the sawtooth voltage signal is required to be synchronized with the frequency of the signal under test. In this way, it is not only required that the frequency of the sawtooth wave voltage can be continuously adjusted, but also a synchronization signal must be input to the circuit that generates the sawtooth wave.
In this way, for a simple oscilloscope that can only produce a continuous scan state, it is necessary to input a synchronization signal related to the frequency of the observed signal to its scan circuit to control the oscillation frequency of the sawtooth wave.
For oscilloscopes with a waiting scan function (that is, no sawtooth wave is generated at ordinary times, a sawtooth wave is generated when the measured signal arrives, and a scan is performed) In order to meet various needs, the synchronization (or trigger) signal can be synchronized or triggered by the signal Select the switch to choose, usually, there are 3 sources.
① The measured signal is drawn from the vertical amplifying circuit as a synchronization (or trigger) signal. This signal is called an internal synchronization (or internal trigger) signal;
②Introduce a certain related external signal as a synchronization (or trigger) signal. This signal is called an external synchronization (or external trigger) signal, which is added to the external synchronization (or external trigger) input terminal;
③Some oscilloscope’s synchronization signal selection switch also has a power synchronization, which is a 220V, 50Hz power supply voltage, which is used as a synchronization signal after being stepped down through the transformer secondary.
Fundamental
Waveform display
According to the principle of the oscilloscope, when a DC voltage is applied to a pair of deflection plates, it will cause a fixed displacement of the light spot on the phosphor screen, and the magnitude of the displacement is proportional to the applied DC voltage. If two DC voltages are applied to the vertical and horizontal deflection plates at the same time, the position of the light spot on the phosphor screen is determined by the displacement in the two directions.
If a sinusoidal AC voltage is applied to a pair of deflection plates, the light spot on the phosphor screen will move with the voltage change. When a sinusoidal AC voltage is applied to the vertical deflection plate, the voltage is Vo (zero value) at the moment of time t=0, and the position of the light spot on the phosphor screen is at the coordinate origin 0. At the moment of time t=1, the voltage is V1 (positive value), the light spot on the fluorescent screen is on the 1 above the coordinate origin 0 points, and the displacement is proportional to the voltage V1;
At the moment of time t=2, the voltage is V2 (the maximum positive value), the light spot on the phosphor screen is at 2 points above the coordinate origin 0 point, and the displacement distance is proportional to the voltage V2; and so on, at time t=3, T=4,…, t=8 at each moment, the positions of the light spots on the phosphor screen are 3, 4,…, 8 points respectively.
In the second cycle, the third cycle of the AC voltage… the situation of the first cycle will be repeated. 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 a light spot moving up and down will be seen on the phosphor screen. The instantaneous deflection value of the light spot from the origin of the coordinate will be proportional to the instantaneous value of the voltage applied to the vertical deflection plate.
Two-line oscilloscope
In the process of electronic practice technology, it is often necessary to observe the process of two (or more than two) signals changing overtime at the same time. And carry on the electric quantity test and comparison to these different signals.
In order to achieve this goal, based on the principle of ordinary oscilloscopes, people use the following two methods to display multiple waveforms at the same time. One is the dual-line (or multi-line) oscillometric method; Multi-track) oscillometric method.
The oscilloscopes manufactured using these two methods are called two-wire (or multi-wire) oscilloscopes and two-track (or multi-track) oscilloscopes.
The dual-line (or multi-line) oscilloscope is realized by using a dual-gun (or multi-gun) oscilloscope tube.
Double trace oscilloscope
The dual-track (or multi-track) oscilloscope is based on the single-line oscilloscope, adding a special electronic switch to realize the separate display of two (or more) waveforms.
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.
The dual-trace oscilloscope is mainly composed of two channels of Y-axis preamplification circuit, gate control circuit, electronic switch, hybrid circuit, delay circuit, Y-axis post-amplification circuit, trigger circuit, scanning circuit, X-axis amplifier circuit, Z Shaft amplifier circuit, calibration signal circuit, oscilloscope, and high and low voltage power supply circuit.
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, they There are suitable and unsuitable places.
Analog
The analog oscilloscope works by directly measuring the signal voltage and drawing the voltage in the vertical direction by an electron beam passing through the oscilloscope screen from left to right.
Digital
The working model of a digital oscilloscope is to convert the measured voltage into digital information through an analog converter (ADC). The digital oscilloscope captures a series of sample values of the waveform and stores the sample values. The storage limit is to judge 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 an analog oscilloscope, oscilloscope, vertical amplification, and horizontal scanning are required to fully advance. 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.
Parameter characteristics
Channel number classification
Generally, whether it is an analog oscilloscope or a digital oscilloscope, it can be divided into single-channel/single-track oscilloscope; dual-channel/dual-track oscilloscope; 2+1 channel (1 external trigger)/three-track oscilloscope; four-channel/four-track oscilloscope 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, each type has many models, but the general oscilloscopes are the same in the basic aspects of use except that the bandwidth and input sensitivity is not completely the same. Take the SR-8 dual-trace oscilloscope as an example.
(1) Analog oscilloscope panel device
The panel device can usually be divided into 3 parts according to its position and function, display, vertical (Y-axis), and horizontal (X-axis). Now introduce the functions of these three parts of the control device respectively.
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 to adjust the sharpness of the light spot or waveform.
(5) Auxiliary focus, adjust the sharpness with the focus knob.
(6) Ruler brightness adjusts the brightness of the scale line on the coordinate sheet.
(7) Tracking, when the button is pressed down, the light spot deviated from the phosphor screen will return to the display area, and the position of the light spot will be found.
(8) Standard signal output 1kHz, 1V square wave calibration signal is derived from this.
Y-axis plug-in part
(1) The display mode selection switch is used to switch the working state of the two Y-axis preamplifiers YA and YB. It has five different display modes.
Alternate, intermittent, YA, YB, YA + YB
The DC-⊥-AC Y-axis input selection switch is used to select the coupling mode of the signal under test connected to the input terminal.
Fine-tuning the V/div sensitivity selection switch and fine-tuning the device.
Balance
(5) ↑↓ Y-axis displacement potentiometer used to adjust the vertical position of the waveform.
(6) Polarity, pull the polarity of the YA A channel, and press the pull switch.
(7) Internal trigger, pull YB trigger source selection switch.
(8) The Y-axis input socket adopts a BNC-type socket, from which the signal to be measured is input directly or through the probe.
X-axis plug-in part
(1) t/div scanning speed selection switch and fine-tuning knob.
Extend and pull ×10 scanning speed expansion device.
Push-pull switch
→← X-axis position adjustment knob
An external trigger, X external socket adopts the BNC socket
(6) Trigger level knob Trigger level adjustment potentiometer knob
(7) Stability triggers the stability fine-tuning knob
(8) Internal and external trigger source selection switch
(9) AC AC (H) DC trigger coupling mode switch
(10) “High frequency, normal state, automatic” trigger mode switch
(11) +,-trigger the polarity switch.
(2) Inspection before use
Before the oscilloscope is used for the first time or when it is stored and reused for a long time, it is necessary to perform a simple check on whether it can work and adjust the stability of the scanning circuit and the DC balance of the vertical amplifying circuit. When the oscilloscope is performing quantitative tests of voltage and time, it must also be calibrated for the gain of the vertical amplifying circuit and the horizontal scanning speed.
The method of checking whether the oscilloscope can work normally, the method of calibrating the gain of the vertical amplifying circuit and the horizontal scanning speed, due to the different parameters such as the amplitude and frequency of the calibration signal of various types of oscilloscope, the inspection and calibration methods are slightly different.
(3) Use steps
The 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 modulation. The following describes the steps of using an oscilloscope to observe the electrical signal waveform.
A.Select the Y-axis coupling method
According to the frequency of the signal under test, set the Y-axis input coupling mode to choose AC-ground-DC switch to AC or DC.
B. 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 coupling mode is DC gear, the superimposed DC voltage value should also be considered), select the Y-axis sensitivity to the V/div switch (or Y-axis attenuation switch) is placed in the appropriate gear.
If you don’t need to read the voltage value in actual use, you can adjust the Y-axis sensitivity fine-tuning (or Y-axis gain) knob appropriately to make the waveform of the required height appear on the screen.
C. Select trigger (or synchronization) signal source and polarity
Usually, the signal polarity switch will be triggered (or synchronized).
D. Select scan speed
According to the approximate value of the measured signal period (or frequency), set the X-axis scanning speed t/div (or scanning range) switch to the appropriate level. If you do not need to read the time value in actual use, you can adjust the sweep speed t/div fine-tuning (or sweep fine-tuning) knob to make the waveform of the number of cycles required for the test displayed on the screen. If what needs to be observed is the edge part of the signal, the sweep speed t/div switch should be set to the fastest sweep speed gear.
E. Input the signal under test
After the measured signal is attenuated by the probe (or directly input by the coaxial cable without attenuation, but at this time the input impedance is reduced and the input capacitance is increased), it is input to the oscilloscope through the Y-axis input terminal.
Common failure phenomena and causes
No light spots or waves
The power is not turned on.
The brightness knob is not well adjusted.
X, Y-axis shift knob position adjustment.
Improper adjustment of the Y-axis balance potentiometer caused a serious imbalance of the DC amplifier circuit.
Can’t expand horizontally
If the trigger source selection switch is placed in the outer 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 to make the scanning circuit in a critical state to be triggered.
The X-axis selection is wrongly placed in the X external position, and there is no signal input on the external socket.
If the two-track oscilloscope only uses channel A (channel B has no input signal), and the internal trigger switch is placed in the YB position, no sawtooth wave is generated.
No vertical display
The input coupling method DC-grounding-AC switch is incorrectly placed in the grounding position.
The high and low potential ends of the input terminal are reversed to the high and low potential ends of the circuit under test.
The input signal is small, and 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).
The trigger coupling mode AC, AC (H), and DC switch failed to correctly select the corresponding gear level according to the different trigger signal frequencies.
When the high-frequency trigger state is selected, the trigger source selection switch is mistakenly set to the outer gear (it should be set to the inner gear.)
When part of the oscilloscope scan is in automatic mode (continuous scan), the waveform is unstable.
Vertical lines are dense or appear as a rectangle
The t/div switch is incorrectly selected, causing f scan<<f signal.
Dense horizontal lines or a slanted horizontal line
Improper selection of t/div off causes f scan >> f signal.
The voltage reading in the vertical direction is inaccurate
No vertical deflection sensitivity (v/div) calibration.
When performing v/div calibration, the v/div fine-tuning knob is not placed in the calibration position (that is, it is not fully rotated in the clockwise direction).
During the test, the v/div fine-tuning knob was moved away from the calibration position (that is, the position where the foot is rotated in a clockwise direction).
Use a 10:1 attenuation probe, and calculate the voltage without multiplying it by 10 times.
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 RMS value needs to be converted.
Inaccurate readings in the horizontal direction
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 fully rotated in the clockwise direction).
During the test, the t/div fine-tuning knob was moved away from the calibration position (that is, the position where the foot is rotated in the clockwise direction).
When the sweep speed extension switch is placed in the pull (×10) position, the test does not increase the sensitivity by 10 times the indicated value of the t/div switch.
The DC voltage value of the AC-DC superimposed signal is unclear
The Y-axis input coupling selection DC-grounding-AC switch is incorrectly placed in the AC position (it should be placed in the DC position).
Before the test, the DC-grounding-AC switch was not placed in the grounding position to calibrate the DC-level reference point.
The Y-axis balance potentiometer has not been adjusted properly.
Can’t detect the phase difference between the two signals
The phase difference between the two signals cannot be measured (waveform display method)
The double-trace oscilloscope mistakenly puts the internal trigger (pull YB) switch in the press (normal state) position and the switch should be put in the YB position.
The dual-trace oscilloscope did not correctly select the alternate and intermittent gears of the display mode switch.
The trigger selector switch of the single-wire oscilloscope is set to the internal gear by mistake.
Although the trigger selection switch of the single-wire oscilloscope is set to the external gear, the same signal is not used for the two external triggers.
Aberration of the amplitude modulation waveform
The t/div switch is incorrectly selected, and the scanning frequency is wrongly selected according to the carrier frequency of the AM wave (should be selected according to the frequency of the audio AM signal).
The waveform cannot be adjusted to the required starting time and position
The stability potentiometer is not adjusted at the critical trigger point to be triggered.
The trigger polarity (+, -) is not properly matched with the trigger level (+, -).
The trigger mode switch is mistakenly placed in automatic gear (should be placed in normal gear).
Triggered or synchronized scan
Slowly adjust the trigger level (or synchronization) knob, and a stable waveform appears on the screen. According to observation needs, adjust the level knob appropriately to display the waveform at the corresponding starting position.
Abnormal phenomena caused by improper use
In the process of using the oscilloscope, often because the operator does not understand the principle of the oscilloscope well and is not familiar with the function of the oscilloscope panel control device, abnormal phenomena may occur due to improper adjustment.
Test applications
Voltage measurement
Any measurement made with an oscilloscope is boiled down to the measurement of voltage. The oscilloscope can measure the voltage amplitude of various waveforms. It can measure both DC voltage and sinusoidal voltage, as well as the amplitude of 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 up-shoot or the amount of top-down. This is unmatched by any other voltage-measuring instrument.
- Direct measurement
The so-called direct measurement method is to directly measure the height of the measured voltage waveform from the screen, and then convert it into a voltage value.
(1) Measurement of AC voltage
(2) Measurement of DC voltage
The direct measurement method is simple and easy to implement, but the error is large. The factors that cause errors include reading errors, parallax, oscilloscope system errors (attenuator, deflection system, oscilloscope edge effect), and so on.
- Comparative measurement
The comparative measurement method is to compare a known standard voltage waveform with the measured voltage waveform to obtain the measured voltage value.
The comparison method can avoid the error caused by the vertical system to measure the voltage, thus improving the measurement accuracy.
Time measurement
The time base of the oscilloscope can generate scan lines that are linearly related to time, so the horizontal scale of the phosphor 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 edge), the time difference between two signals, etc.
Phase measurement
It is of practical significance to use an oscilloscope to measure the phase difference between two sinusoidal voltages. A counter can measure frequency and time, but it cannot directly measure the phase relationship between sinusoidal voltages.
Frequency measurement
There are many ways to measure the frequency of a signal with an oscilloscope. The following two basic methods are commonly used.
- Cycle 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
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.
- Graphic method to measure the frequency
Set the oscilloscope to X-Y working mode, input the measured signal into the Y-axis, and externally connect the standard frequency signal input X, and slowly change the standard frequency so that the two signal frequencies become integer multiples, for example, 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 related to the frequency of the two deflection voltages. The tracing method can be used to draw graphs of various frequency ratios and different phase differences between UX and uy.
Using the relationship between graph and frequency, accurate frequency comparison can be performed to determine the frequency of the signal under test. The method is to draw horizontal and vertical lines through the graphics respectively, and the horizontal and vertical lines drawn should not pass through the intersection of the graphics 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, in order to make the test simple and correct, when conditions permit, the frequency of the known frequency signal is usually adjusted as much as possible when the Lissajous figure is used for the frequency test so that the figure displayed on the phosphor screen is a circle or an ellipse. At this time, the frequency of the signal under test is equal to the frequency of the known signal.
Since the two voltages applied to the oscilloscope have different phases, the graphics on the phosphor screen will have different shapes, but this has no effect on determining the unknown frequency.
Oscilloscopes are divided into universal oscilloscopes, digital oscilloscopes, analog oscilloscopes, virtual oscilloscopes, arbitrary waveform oscilloscopes, handheld oscilloscopes, digital phosphor oscilloscopes, and data acquisition oscilloscopes.
You may also be interested in below other C&T RF Antennas Inc‘s antenna test equipment: