1. Periodic method
For any periodic signal, the time interval can be used to measure the aforementioned method, the first determination of the time of each cycle T, and then use the following formula to find the frequency f: f=1/T

For example, the oscilloscope shows the measured waveform, a cycle of 8div, "t/div" switch set to "1μs" position, the "fine-tuning" set to "Calibration" position. The 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. Li Shayu graphical method of frequency measurement
Oscilloscope set X-Y mode of operation, the signal under test into the Y-axis, the standard frequency signal input "X external", slowly change the standard frequency, so that the two signals become integer multiples of the frequency, for example, fx: fy=1:2.

fy=1:2, then the fluorescent screen will form a stable Lishayu graphics.

The shape of the Lissajous pattern is not only related to the phase of the two deflection voltages, but also related to the frequency of the two deflection voltages. With the tracing method can be drawn ux and uy a variety of frequency ratios, different phase difference when the Lishayu graph, several different frequency ratios of the Lishayu graph shown in Figure 5-15.

fy/fx=m/n

The LISAYO graphical method of measuring frequency is quite accurate, but the operation is more time-consuming. At the same time, it is only suitable for measuring signals with lower frequencies.