An oscilloscope is an indispensable instrument in modern electrical repair. With it, technicians can quickly and accurately locate common faults, making the proper and skilled use of an oscilloscope a required skill for electrical repair personnel.
Although there are various models, specifications, and types of oscilloscopes, their basic structure and function are quite similar. This article provides a detailed introduction to the operation methods of a general-purpose oscilloscope.
1. Control Panel Overview
1. Brightness and Focus Knobs
The brightness adjustment knob is used to adjust the brightness of the trace (some oscilloscopes refer to this as “luminance”). When using it, the brightness should be appropriate; if too bright, it may easily damage the cathode ray tube. The focus adjustment knob is used to adjust the clarity (size) of the trace. It should be adjusted until the waveform is clear.
2. Signal Input Channels
Common oscilloscopes usually have dual trace capabilities, with two input channels, CH1 and CH2. Each can connect to an oscilloscope probe, and the oscilloscope’s casing should be grounded. The probe should be inserted at the measurement point for accurate measurement.
3. Channel Selection Buttons (Vertical Mode Selection)
Common oscilloscopes have five channel selection buttons:
(1) CH1: Independent display of channel 1;
(2) CH2: Independent display of channel 2;
(3) ALT: Alternating display of two channels;
(4) CHOP: Interleaved display of two channels, used for slow scanning dual trace display;
(5) ADD: Signal addition of both channels. Typically, channel 1 or channel 2 is selected for repairs.
4. Vertical Sensitivity Adjustment Knob
This knob adjusts the vertical offset sensitivity. It should be adjusted according to the strength of the input signal. The value indicated on the knob (e.g., 0.9V/div) indicates that each division on the vertical axis represents 0.9V. Multiply this value by the number of divisions occupied by the measured signal on the display to obtain the signal strength.
5. Vertical Position Adjustment Knob
This is used to adjust the position of the measured signal trace on the vertical axis of the display.
6. Horizontal Scan Rate Adjustment Knob
This knob adjusts the horizontal rate. It should be set according to the frequency of the input signal. The value indicated on the knob (e.g., 0.5ms/div) indicates that each division on the horizontal axis represents 0.5ms. Multiply this value by the number of divisions occupied by one cycle of the measured signal to obtain the signal period, which can also be converted into frequency.
7. Horizontal Position Adjustment Knob
This is used to adjust the position of the measured signal trace on the horizontal axis of the display.
8. Trigger Mode Selection
Oscilloscopes generally have four trigger modes:
(1) Normal (NORM): No display on the screen when there is no signal; when there is a signal, a stable waveform is displayed in coordination with the pulse signal control;
(2) Auto (AUTO): The trace is displayed on the screen when there is no signal; when there is a signal, a stable waveform is displayed in coordination with the pulse signal control;
(3) TV: Used to display television signals;
(4) Peak Detect (P-PAUTO): The trace is displayed on the screen when there is no signal; when there is a signal, a stable waveform is displayed without adjusting the pulse signal. This mode is only available on some oscilloscopes (e.g., CALTEK’s CA8000 series oscilloscopes).
9. Trigger Source Selection
Oscilloscopes have two types of trigger sources: internal and external. If an external trigger source is selected, the trigger signal must be input from the external trigger input terminal. This method is rarely used in appliance repair. If an internal trigger source is selected, channel 1 (CH1) or channel 2 (CH2) should be chosen according to the input signal channel. If channel 1 is selected for the input signal, the internal trigger source should also be set to channel 1.
2. Measurement Methods
1. Measurement of Voltage and Frequency (Using the Oscilloscope’s Calibration Signal as an Example)
(1) Insert the oscilloscope probe into the channel 1 socket and set the probe’s attenuation to the “1” position;
(2) Set the channel selection to CH1 and the coupling method to DC;
(3) Insert the probe tip into the calibration signal source’s small hole, and the oscilloscope screen will display a trace;
(4) Adjust the vertical and horizontal knobs to stabilize the waveform displayed on the screen, placing the vertical and horizontal adjustments in the calibration direction;
(5) Read the number of divisions occupied by the waveform on the vertical axis, and multiply it by the value indicated on the vertical attenuation knob to obtain the calibration signal’s strength;
(6) Read the number of divisions occupied by each cycle of the waveform on the horizontal axis, and multiply it by the value indicated on the horizontal scan knob to obtain the calibration signal’s period (the period can be converted to frequency);
(7) The calibration signal typically has a frequency of 1kHz and a strength of 0.9V, used to calibrate the internal scanning oscillator frequency of the oscilloscope. If there are anomalies, the oscilloscope’s (internal) resistors should be adjusted until they match.
2. Example of Oscilloscope Use (Measuring the 13MHz Pulse Generator in the 788 Mobile Phone)
The normal operation of the 13MHz digital clock signal in the mobile phone is essential for its startup. Therefore, it is important to frequently measure the presence of the 13MHz digital clock signal during repairs. The process is as follows:
(1) Turn on the oscilloscope, adjust the brightness and focus knobs to display a horizontal bright line with appropriate brightness and good focus;
(2) Calibrate the oscilloscope as described above, then set the coupling method to AC;
(3) Attach the ground clip of the oscilloscope probe to the mobile phone’s circuit board ground, and insert the probe into pin 13 of the 788 mobile phone CPU;
(4) Connect the mobile phone’s power supply and press the power button. Adjust the vertical and horizontal scan knobs, observing whether a stable waveform appears on the display. If not, it generally indicates the absence of the 13MHz signal.