Introduction to the method of measuring frequency with an oscilloscope
1. Periodic method
For any periodic signal, the aforementioned time interval measurement method can be used to first measure the time T of each cycle, and then use the following formula to find the frequency f: f=1/T
For example, the measured waveform displayed on the oscilloscope has a period of 8div, the "t/div" switch is set to the "1μs" position, and its "fine adjustment" is set to the "calibration" position. Then its period and frequency are calculated as follows:
T=1us/div×8div = 8us
f= 1/8us =125kHz
Therefore, the frequency of the measured waveform is 125kHz.
2. Frequency measurement using Lissajous figure method
Set the oscilloscope to X-Y working mode, input the measured signal to the Y axis, and input the standard frequency signal to "X external". Slowly change the standard frequency until the frequency of the two signals is an integer multiple, such as fx:
If fy=1:2, a stable Lissajous figure will be formed on the fluorescent screen.
The shape of the Lissajous figure is not only related to the phases of the two deflection voltages, but also to the frequencies of the two deflection voltages. The tracing method can be used to draw Lissajous figures at various frequency ratios and different phase differences between ux and uy. Several Lissajous figures with different frequency ratios are shown in Figure 5-15.
Using the relationship between Lissajous figure and frequency, accurate frequency comparison can be made to determine the frequency of the signal being measured. The method is to draw horizontal and vertical lines through the Lissajous figure respectively. The horizontal and vertical lines should not pass through the intersection of the figure or be tangent to it. If the number of intersection points of the horizontal line and the figure is m, and the number of intersection points of the vertical line and the figure is n, then
fy/fx=m/n
When the standard frequency fx (or fy) is known, the measured signal frequency fy (or fx) can be obtained from the above formula. Obviously, in actual test work, when using Lissajous graphics for frequency testing, in order to make the test simple and correct, if conditions permit, the frequency of the known frequency signal is usually adjusted as much as possible so that the graphics displayed on the fluorescent screen are circles or ellipses. . At this time, the frequency of the measured signal is equal to the frequency of the known signal.
