4.3.3 Experimental principles
An electronic oscilloscope is a commonly used electronic measuring instrument.It can display the electrical process that changes through seconds with images.It is able to observe and measure the size,frequency,phase,and waveform of electrical signals,observe the subtle processes that occur in a very short time and measure the voltage in the order of microvolts.It can also detect and analyze two signals at the same time.It is an indispensable instrument in modern electrical measurement,and has a wide range of applications in various fields such as industrial and agricultural production,military,scientific research,and teaching.
Oscilloscopes can be roughly divided into general-purpose oscilloscopes,sampling oscilloscopes,memory and storage oscilloscopes,and special-purpose oscilloscopes according to their use and technical performance.In this experiment,we will learn to use the dual trace oscilloscope as the object.
1)Introduction to the principle of universal oscilloscope
At present,there are many types of oscilloscopes for measurement with complex structures,but all of oscilloscopes contain the following basic components:oscilloscope tube(i.e.,cathode ray tube,CRT),amplifier,scan generator,trigger synchronization and power supply.Figure 4.3.3 shows the composition principle.
(1)The basic structure of the oscilloscope tube:The oscilloscope tube is the core component of the oscilloscope.It realizes the transformation from an electrical signal to an optical signal.It projects the high-speed moving electron beam on the phosphor screen under the action of the deflection system to make the phosphor screen produce a bright spot,using the measured voltage and scanning the voltage acts on the electron beam to form a bright line on the phosphor screen.So that the signal waveform can be displayed.
Figure 4.3.3 Schematic diagram and compositions of the electrical oscilloscope
The oscilloscope tube is an electronic vacuum device composed of three parts:electron gun,deflection system and phosphor screen.The structure diagram is shown in Figure 4.3.4.
Figure 4.3.4 Schematic diagram of oscilloscope tube
F—filament;K—cathode;G—control grid;A1—first anode;A2—second anode;
Y—vertical deflection plate;X—horizontal deflection plate
①Electron gun:It is composed of filament F,cathode K,control grid G,anode A1 and A2.The filament is energized to heat the cathode and an electron beam is emitted after the cathode temperature rises.The potential of the control grid G is lower than that of the cathode K,which has a certain control effect on the movement of the electron beam.Adjust the brightness knob on the oscilloscope is to adjust the potential of the grid and control the number of electrons emitted to the fluorescent screen.Then change the brightness of the light spot on the fluorescent screen.When the electron beam reaches the anode,the electrons are accelerated by the electric field between them to form rays since the potential of the anode is much larger than the potential of the cathode,which hit the phosphor screen to form small dots.Both the first A1and second anode A2have a focusing effect,among which A1is called the focusing anode and A2is called the accelerating anode(or auxiliary focusing).
②Deflection system:It consists of two pairs of metal plates X(horizontal deflection plate)and Y(vertical deflection plate)that are perpendicular to each other.Voltage is applied to the two pairs of deflection plates.When the electron beam enters the deflection plate at a certain speed,it will be deflected by the force of the electric field and the position on the phosphor screen will be deflected.If the voltage of the two pairs of deflection plates is periodic changes,the electron beam will form bright lines on the phosphor screen.
③Fluorescent screen:The inner wall is coated with fluorescent material at one end of the oscilloscope tube.When the electron beam hits the fluorescent screen,it will cause fluorescence to form a light spot.
(2)Amplifier:The amplification in the oscilloscope is divided into two sets of systems:vertical amplifier and horizontal amplifier.
On the oscilloscope panel is the V/div knob.When the voltage of the signal to be measured is small,the Y axis amplifier is used for vertical amplification;when the voltage of the signal to be measured is large,the Y axis attenuator is used for vertical attenuation;The deflection voltage is amplified or attenuated to an appropriate size,so that the signal can be expanded horizontally,which is also called scanning.
(3)Waveform display mechanism of the oscilloscope:Apply a periodic voltage to the vertical deflection plate,and we can see a vertical bright line on the phosphor screen.If a scanning voltage is applied to the horizontal deflection plate at this time,the voltage is a sawtooth voltage,that is,the voltage increases to a certain value through time.It suddenly returns to minimum,and then starts to increase again.Repeatedly,when the period of the voltage applied on the X and Y axes is the same,the same complete waveform signal will be displayed on the fluorescent screen.
The principle diagram of the waveform displayed by the oscilloscope is shown in Figure 4.3.5.The signal of X direction is a sawtooth wave and the signal of Y direction is a sine wave.It can be seen that the waveform seen on the phosphor screen is actually a trajectory composed of two mutually perpendicular motions.The electron beam is affected by Uy.The action is deflected upward.And at the same time,it is deflected to the right by the action of Ux.That is to say,Uy determines the y coordinate of the bright spot and Ux determines the x coordinate of the bright spot.
(4)The concept of synchronization:When the periods of the X sawtooth wave and the Y axis signal are not equal or integer multiples,the oscilloscope display principle knows that the waveform displayed on the fluorescent screen is unstable,because the starting point of the synthesized waveform of the two cycles before or after does not coincide the waveform occurs movement.Therefore,in order for the oscilloscope to display stable waveforms,the voltage cycles of the X and Y axes must be synchronized-equal or integer multiples.To adjust the synchronization on the oscilloscope panel is to adjust the LEVEL knob.
Figure 4.3.5 Schematic diagram of displaying waveform in an oscilloscope
2)Dual-track display principle and synchronization requirements
The dual trace oscilloscope has two independent input channels(set to CH1 and CH2 channels).Through the switch of the electronic switch,the signals of the two channels are sent to the common Y axis amplifier in turn to achieve dual trace display.There are two ways to switch the electronic switch:“Alternate”(ALT)and“Intermittent”(CHOP).When in the“alternating”mode,the switching speed of the electronic switch is controlled by the scanning rate.For example,in the first scan,the switch turns on the CH1 channel,displaying the waveform of channel A.In the second scan,the switch turns on the CH2 channel,displaying the waveform of the CH2 channel.In the third scan,the switch turns on the CH1 channel again….Obviously,the higher the frequency of the measured signal,the higher the scanning rate,and the faster the switching speed of the electronic switch.Due to the afterglow effect of the fluorescent screen,two clear waveforms are obtained on the screen,so this method is suitable for higher frequency signal waveform display.In the“intermittent”mode,the electronic switch is not controlled by the scanning signal,and its switching frequency is fixed at 200 kHz.It is equivalent to sampling and displaying the A and B channel signals in turn at a sampling rate of 200 kHz in each scan,so the displayed waveform is intermittent.Especially when the signal frequency is high,the reduction of sampling points is more obvious,so this method is suitable for displaying lower frequency signal waveforms.
For dual trace display,in order to obtain two stable and phase-correct waveforms,there are often higher synchronization requirements.First,one of the two signals must be selected as the trigger signal,and this signal is used to trigger the signal to scan.The conditions suitable for triggering signals are:
①The one with a long period.
②The one with time leading.
③The one with a steep waveform edge,large amplitude and few wave numbers.
3)Lissajous figures(https://www.daowen.com)
If sine waves with a frequency of a simple integer ratio is added to the X and Y axes deflection plates of the oscilloscope,the result of the synthesized vibration is a closed curve,and the Lissajous figure shown in Figure 4.3.6 can be obtained on the fluorescent screen.
Figure 4.3.6 Lissajous figures
The Lissajous figure can be used to measure another unknown frequency from a known frequency or the phase difference between two vibrations.The specific method of measuring the unknown frequency is:if the known frequency is fx and the unknown frequency is fy.Input fx signal to the X axis,and fy signal to the Y axis.If the ratio is an integer,you will get one stable Lissajous graphics on the fluorescent screen.If the graph has Nx intersections on the X axis and Ny intersections on the Y axis,there is:
Thus,the unknown frequency is obtained as:
In the formula,the number of intersections Nx and Ny are the largest number of intersections with X axes and Y axes.
4)Measurement example
When quantitatively measuring waveform parameters,the sensitivity of the X and Y channels are both at the correction position by fine-tuning.
(1)Peak-to-peak voltage VP-P measurement
Input the signal to a channel,adjust the vertical deflection sensitivity,scanning speed selection switch and level knob to get a stable and appropriate waveform.
If the waveform shown in Figure 4.3.7 is displayed on the fluorescent screen,the peak-topeak voltage value VP-P of the measured signal is:
Effective voltage:
Using data in Figure 4.3.7 as example:Y=4(div),Ky=2(V/div),vertical expansion factor is 1,and then VP-P=4×2=8(V).
(2)DC voltage measurement
Set AC-GND-DC switch to the“GND”position,display the baseline during scanning in automatic scanning mode,and adjust the vertical displacement knob to make it on a certain horizontal coordinate,the time baseline displayed at this time is zero level refer to the baseline.
When AC-GND-DC is placed in the“DC”position,the scanning line moves up or down,and its moving value is the DC voltage value V:
(3)Period and frequency measurement
As shown in Figure 4.3.7,the period T of the waveform is:
Figure 4.3.7 Peak to peak voltage
In this figure,X=2(div),KS=5 ms/div,horizontal expansion factor is 1,and T=2×5/1=10(ms).
