The use and measurement methods of general oscilloscope

　　An oscilloscope is an electronic measuring instrument that converts analog electrical signals into waveforms to display signal images. Through the waveform of the circuit being tested, we can observe the amplitude, phase, and frequency of the waveform, and we can also analyze and compare the waveform. Although there are many brands and models of oscilloscopes with various functions, the use and operation process of oscilloscopes are basically similar. The following is an introduction to the use of general oscilloscopes.

　　1. 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:

　　(1) CH1: Channel 1 is displayed separately;

　　(2) CH2: Channel 2 is displayed separately;

　　(3) ALT: two channels are displayed alternately;

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

　　(5) 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 vertical grid 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 horizontal grid time 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:

　　(1) 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;

　　(2) 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;

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

　　(4) 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 used in some oscilloscopes (such as CALTEK CA8000 series oscilloscopes).

　　9. Trigger source selection

　　There are two types of trigger sources for oscilloscopes: internal trigger source and external trigger source. If you choose an external trigger source, the trigger signal should be input from the external trigger source input terminal, which is rarely used in home appliance maintenance. If you choose an internal trigger source, generally select channel 1 (CH1) or channel 2 (CH2), which should be selected according to the input signal channel. If the input signal channel is selected as channel 1, the internal trigger source should also select channel 1.

　　2. Measurement Method

　　1. Amplitude and frequency measurement method (taking the calibration signal of the test oscilloscope as an example)

　　(1) 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, and 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 obtain the cycle of the calibration signal (the inverse of the cycle 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.

　　2. Oscilloscope application example (taking the measurement of 13MHz clock pulse of 788 mobile phone as an example)

　　The normal 13MHz clock signal in the mobile phone is a necessary condition for starting up, so when repairing, you should often measure whether there is a 13MHz clock signal. The steps are as follows:

　　(1) Turn on the oscilloscope and adjust the brightness and focus knobs so that a horizontal bright line with moderate brightness and good focus appears on the screen;

　　(2) Calibrate the oscilloscope as described above, and then set the coupling mode to AC.

　　(3) Clamp the grounding clip of the oscilloscope probe to the grounding point of the mobile phone circuit board, and insert the probe into the 788 mobile phone CPU pin;

　　(4) Turn on the phone, press the power button, adjust the vertical scan level and horizontal scan knob, and observe whether a stable waveform appears on the screen. If not, it generally means that there is no 13MHz signal.