“The analog oscilloscope uses an analog circuit (oscilloscope tube, 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 a fluorescent substance, so that the electrons The spot the beam hits will emit light. The way it works is that the signal voltage is measured directly and the voltage is plotted vertically by the electron beam passing through the oscilloscope screen from left to right.

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The analog oscilloscope uses an analog circuit (oscilloscope tube, 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 a fluorescent substance, so that the electrons The spot the beam hits will emit light. The way it works is that the signal voltage is measured directly and the voltage is plotted vertically by the electron beam passing through the oscilloscope screen from left to right.

Analog Oscilloscope Features

Analog oscilloscopes are inherently capable of probabilistic Display. Due to the persistence of afterglow on the screen, waveform events with different probabilities will appear on the screen with different brightness. It has a certain persistence and cannot be displayed. Probability Display is a very useful function. For example, there is a glitch on a waveform that does not appear every time. If you use DSO, the display of this glitch will keep shaking. If you pause the display, there may be no glitch, or there may be no glitch. If there is a burr, you can’t judge the probability of the burr. If you use ART, the probability of the burr will be displayed with different brightness. Because of this feature, in the field of switching power supply development, analog oscilloscopes are widely used at low prices.

The principle of analog oscilloscope

The core component of the analog oscilloscope is the cathode ray oscilloscope tube. The electron gun emits an electron beam, and the electron beam is directed to the phosphor screen after passing through the Y deflection plate and the X deflection plate. The schematic diagram of the device is shown in Figure 1.

Figure 1 Cathode ray oscilloscope

(1) When the signal to be tested is applied to the Y deflection plate and no voltage is applied to the X-axis deflection plate, a bright spot will be displayed on the leftmost ordinate of the phosphor screen, and the higher the voltage, the higher the bright spot.

(2) When the sawtooth wave signal is applied to the X deflection plate and no voltage is applied to the Y deflection plate, with the rise of the sawtooth wave voltage, the light spot moves uniformly from the far left of the screen to the far right of the screen. When the sawtooth wave starts from zero, it moves rapidly from the far left.

Therefore, for the analog oscilloscope, the signal to be tested should be applied on the Y deflection plate, and the sawtooth wave signal (scanning signal) should be applied on the X deflection plate. But the above description alone is not enough to stably display the signal waveform on the oscilloscope screen. When the X-axis deflection plate sawtooth wave is automatically triggered, due to the uncertainty of the trigger position, the waveform seen by the observer on the oscilloscope seems to be rolling, and multiple waveforms overlap.

Figure 2 Waveform scrolling caused by automatic triggering

To solve this problem, the trigger concept is introduced. That is, under what trigger conditions are met to open the sawtooth wave on the X deflection plate? There is a trigger level knob (level) on the analog oscilloscope and the ability to select rising/falling edge (slope) triggering. When a DC trigger voltage is determined and rising edge trigger is selected, the graph on the oscilloscope screen is shown in Figure 3.

Figure 3 Influence of trigger level on waveform display

It can also be seen from Figure 3 that the X-axis sawtooth wave cycle determines the size of the horizontal-axis time base. When the period of the sawtooth wave is longer (the width of the triangular part is wider), the time base (time/div, the time represented by each division of the horizontal axis) is larger, and the number of waveform periods displayed on the display screen is larger.