Use of oscilloscope and Lissajous figures

1. Working principle of oscilloscope: Using a narrow electron beam composed of high-speed electrons to hit a screen coated with fluorescent material, tiny light spots can be generated (working principle of traditional analog oscilloscope). Oscilloscope is a widely used electronic measuring instrument. It can transform invisible electrical signals into visual images, which is convenient for studying the changing process of various electrical phenomena.

2. Panel Introduction

(1) Brightness and focus knob

The brightness adjustment knob is used to adjust the brightness of the light trace (some oscilloscopes call it "brightness"). The brightness should be appropriate when using it. If it is too bright, it is easy to damage the oscilloscope tube. The focus adjustment knob is used to adjust the focus (coarseness) of the light trace. It is better to use it when the image is clear.

(2) Signal input channel

Commonly used oscilloscopes are mostly dual-trace oscilloscopes, which have two input channels, namely Channel 1 (CH1) and Channel 2 (CH2). They can be connected to oscilloscope probes respectively, and then the oscilloscope casing is grounded and the probes are inserted into the part to be measured for measurement.

(3) Channel selection key (vertical selection)

Common oscilloscopes have five channel selection keys:

CH1: Channel 1 is displayed separately;

CH2: Channel 2 is displayed separately;

ALT: Two channels are displayed alternately;

CHOP: two-channel intermittent display, used for dual-trace display when the scanning speed is slow;

ADD: Signals from two channels are added together. Channel 1 or channel 2 is usually selected during maintenance.

(4) Vertical sensitivity adjustment knob

To adjust the vertical deflection sensitivity, the position of the knob should be adjusted according to the amplitude of the input signal. Multiply the value indicated by the knob (such as 0.5V/div, indicating that the amplitude of each grid in the vertical direction is 0.5V) by the number of grids occupied by the measured signal in the vertical direction of the screen to obtain the amplitude of the measured signal.

(5) Vertical movement adjustment knob

Used to adjust the vertical position of the measured signal light trace on the screen.

(6) Horizontal scan adjustment knob

To adjust the horizontal speed, the knob position should be adjusted according to the frequency of the input signal. Multiply the knob value (such as 0.5ms/div, indicating that each grid in the horizontal direction is 0.5ms) by the number of grids occupied by one cycle of the measured signal to obtain the period of the signal, which can also be converted into frequency.

(7) Horizontal position adjustment knob

Used to adjust the horizontal position of the measured signal light trace on the screen.

(8) Trigger mode selection

Oscilloscopes usually have four trigger modes:

Normal state (NORM): When there is no signal, there is no display on the screen; when there is a signal, it cooperates with the level control to display a stable waveform;

AUTO: When there is no signal, a light trace is displayed on the screen; when there is a signal, it cooperates with the level control to display a stable waveform;

TV field (TV): used to display TV field signals;

Peak value automatic (PP AUTO): When there is no signal, a light trace is displayed on the screen; when there is a signal, a stable waveform display can be obtained without adjusting the level. This mode is only available on some oscilloscopes.

3. Lissajous Figures

(1) Turn on the oscilloscope, insert the oscilloscope probe into the channel 1 jack, and set the attenuation on the probe to level "1";

(2) Set the channel selection to CH1 and the coupling mode to DC;

(3) Insert the probe tip into the small hole of the calibration signal source. A light trace will appear on the oscilloscope screen.

(4) Adjust the vertical knob and horizontal knob to stabilize the waveform displayed on the screen, and place the vertical fine-tuning and horizontal fine-tuning in the calibration position;

(5) Read the number of grids occupied by the waveform in the vertical direction, multiply it by the indicated value of the vertical attenuation knob to obtain the amplitude of the calibration signal;

(6) Read the number of grids occupied by each cycle of the waveform in the horizontal direction, multiply it by the indicated value of the horizontal scan knob, and get the period of the calibration signal (the reciprocal of the period is the frequency);

(7) The frequency of the general calibration signal is 1kHz and the amplitude is 0.5V, which is used to calibrate the internal scanning oscillator frequency of the oscilloscope. If it is abnormal, the corresponding potentiometer of the oscilloscope (internal) should be adjusted until it matches.

(8) The calibration method for channel 2 is the same as that for channel 1, so calibrate channel 2 using the same method.

(9) Turn on the signal generator and connect the signal generator to the oscilloscope. Set the oscilloscope to the xy working mode.

(10) When the standard frequency is slowly changed so that the two signal frequencies become integer multiples, a stable Lissajous figure will be formed on the screen.