The structure and working principle of dual-trace oscilloscope

Measurement of AC signal with DC component
The oscilloscope can measure AC signal with DC component, including the size of DC component,  the size of AC component and the instantaneous value of a certain point of AC signal. Measurement process of AC signal with DC component. The measurement process
of AC signal with DC component is as follows:
Steps 1 to 4: Set the input coupling mode conversion switch to "丄", set the trigger polarity selection switch to "+", set the trigger signal source selection switch to "In", and then turn the trigger level knob to the "Auto" position. At this time, a horizontal scan line will appear on the screen.
Step 5: Estimate the voltage value of the measured signal, select the appropriate vertical sensitivity gear through the V/div switch, and turn the fine-tuning knob on V/div clockwise to the bottom.
Step 6: Estimate the frequency of the measured signal, select the appropriate horizontal scan , and turn the fine-tuning knob on V/div clockwise to the bottom. .Step
7: Adjust the vertical shift knob to move the horizontal scan line to the appropriate position as  the zero reference line (0V).
Step 8: Switch the input coupling mode switch to the "DC" position.
Step 9: Connect the 10:1 probe to the circuit under test.
Step 10: Observe the waveform of the measured signal on the screen and calculate various values ​​based on the waveform on the screen. If the signal on the screen is unstable, adjust the trigger level knob to stabilize the graph. Next, calculate the DC and AC components of the signal and the instantaneous value of a certain AC point.
(1) Calculation of DC component
In the figure, the DC component level is 3div away from the zero reference level,  the V/div switch  position is 0.2V/div, and the probe attenuation is 10:1, so the voltage of the DC component is  Udirect = 3divx0.  2V/divx10=6V.
(2) Calculation of AC component
In the figure, the positive peak and negative peak of the AC component are 2div away, the V/div switch position is  0.2V/div, and the probe attenuation is 10:1, so the peak-to-peak value of the AC component is = 2div  x  0.  2V/divx10=4V.
(3) Calculation of the instantaneous value of point A of the AC signal
In the figure, the distance between point A of the AC signal and the zero reference level is 4div,  the V/div switch position is 0.2V  /div, and the probe attenuation is 10:1. Then the instantaneous value of point A of the AC signal is Uk=  4div  x  0.2V  /div  x  10  =  8V.
In the actual measurement process, it is often necessary to observe two (or more) signals with the same frequency at the same time to facilitate comparison and analysis. This requires the use of a dual-trace (or multi-trace) oscilloscope . The principles of multi-trace oscilloscopes and dual-trace oscilloscopes are basically the same, but dual-trace oscilloscopes are more widely used. Therefore, this section mainly discusses the principles and usage of dual-trace oscilloscopes.
1. Structure of dual-trace oscilloscopes
There are two main types of dual-trace oscilloscopes: one is an oscilloscope using a dual-beam oscilloscope tube; the other is an oscilloscope using a single-beam oscilloscope tube.
The dual-trace oscilloscope of the dual-beam oscilloscope uses a dual-beam oscilloscope, as shown in Figure 7-30. There are two electron guns and deflection plates inside. They are independent of each other, but share a fluorescent screen. When  measuring, as long as the two signals are sent to their respective deflection plates, the electron beams emitted by the two electron guns will respectively scan out two signal waveforms at different positions on the fluorescent screen. The dual-  trace oscilloscope of the single-beam oscilloscope uses the same oscilloscope as the single-trace oscilloscope. Since this oscilloscope has only one  electron gun, in order to display two signal waveforms on the fluorescent screen at the same time, it is necessary to achieve it through conversion.
The dual-trace oscilloscope of the dual-beam oscilloscope uses a high-cost dual-beam oscilloscope
and requires two sets of deflection circuits and Y channels. Therefore, it has  the advantages of less interference, convenient adjustment of each signal, clear and bright waveform display, and small measurement error during measurement. However, due to its high price and high power consumption, its popularity is far less than that of the dual-trace oscilloscope of the single-beam oscilloscope.
Here we mainly introduce the widely used dual-trace oscilloscope of single-beam oscilloscope
2. Principle of multi-waveform display
Single-beam oscilloscope has only one electron gun. To realize the display of two waveforms on one screen, two scanning methods can be used: one is alternating conversion scanning; the other is intermittent conversion scanning.
(1) Alternating conversion scanning
Alternating conversion scanning is to scan out the measured signal of one channel in one cycle of the scanning signal (silver tooth wave voltage), and scan out the measured signal . Explain the principle of alternating conversion scanning.
When the sawtooth wave voltage during 0~t2 is sent to the X deflection plate, the electronic switch is set to  "1", and the A segment of the UY1 signal of the Y1 channel is sent to the Y deflection plate through the switch, and the A segment of the UY2 signal is scanned on the screen.
When the sawtooth wave voltage during t2~t4 is sent to the X deflection plate, the electronic switch is switched to  "2", and the B segment of the UY2 signal of the Y2 channel is sent to the Y deflection plate through the switch, and the B segment of the UY2 signal is scanned on the screen  . When the sawtooth voltage during t4-t6 is sent to the X deflection plate, the electronic switch is switched to "1" again, and the C segment of the UY1 signal of the Y1 channel is sent to the Y deflection plate through the switch, and the C segment of the Uy1 signal is scanned out on the screen. In this way, the waveforms of the UY1 and UY2 signals are scanned out on the screen in sequence, and the two signals will be displayed one after another, but due to the afterglow effect of the phosphor, the UY2 signal waveform is still displayed after it is scanned out, so the signal waveforms of the two channels can be seen on the screen at the same time.