Introduction to direct measurement method with oscilloscope
The so-called direct measurement method is to measure the height of the measured voltage waveform directly from the screen and then convert it into a voltage value. When quantitatively testing the voltage, generally turn the fine-tuning knob of the Y-axis sensitivity switch to the "calibration" position. In this way, the measured value can be directly calculated from the indicated value of "V/div" and the vertical axis coordinate value occupied by the measured signal. Voltage value. Therefore, the direct measurement method is also called the ruler method.
(1) Measurement of AC voltage
Set the Y-axis input coupling switch to the "AC" position to display the AC component of the input waveform. If the frequency of the AC signal is very low, the Y-axis input coupling switch should be placed in the "DC" position.
Move the measured waveform to the center of the oscilloscope screen, use the "V/div" switch to control the measured waveform within the effective working area of the screen, and read the Y-axis occupied by the entire waveform according to the graduations of the coordinate scale. The degree of direction H, then the peak-to-peak value VP-P of the measured voltage can be equal to the product of the "V/div" switch indication value and H. If a probe is used for measurement, the attenuation of the probe should be taken into account, that is, the above calculated value should be multiplied by 10.
For example, the Y-axis sensitivity switch "V/div" of the oscilloscope is at the 0.2 level, and the Y-axis coordinate amplitude H of the measured waveform is 5div, then the peak-to-peak value of this signal voltage is 1V. If measured by the probe, the above value is still indicated, then the peak-to-peak value of the measured signal voltage is 10V.
(2) Measurement of DC voltage
Set the Y-axis input coupling switch to the "ground" position and the trigger mode switch to the "auto" position so that the screen displays a horizontal scan line, which is a zero-level line.
Set the Y-axis input coupling switch to the "DC" position and add the measured voltage. At this time, the scanning line generates a jump displacement H in the Y-axis direction. The measured voltage is the product of the "V/div" switch indication value and H.
The direct measurement method is simple and easy to implement, but the error is large. Factors causing errors include reading error, parallax, and oscilloscope system errors (attenuator, deflection system, oscilloscope tube edge effect), etc.
