What are the methods for oscilloscope to measure signal frequency?

The oscilloscope is an electronic instrument that electronic engineers use frequently for daily testing and measurement. It is known as the "eyes" of electronic engineers. It has many functions, such as:

1. The period of the AC signal can be measured and used to calculate the frequency of the AC signal.

2. Can measure the voltage amplitude of DC signal and AC signal

3. Can display the waveform of AC signal.

4. Signal measurement can be performed using two channels separately.

5. The waveforms of two signals can be displayed on the screen at the same time, which is the dual-trace measurement function. This function can measure the phase difference between the two signals and the difference in shape between the waveforms.

As a novice engineer, it is very necessary to understand the test methods of each application. Today, Antai Test will share with you how to measure signal frequency with an oscilloscope. There are many ways to measure signal frequency with an oscilloscope. The following are two basic methods commonly used:

1. Periodic method

For any periodic signal, the aforementioned time interval measurement method can be used to first determine the time T of each cycle, and then use the following formula to calculate the frequency f: f=1/T

For example, if the waveform being measured displayed on the oscilloscope has a cycle of 8 div, the "t/div" switch is set to the "1μs" position, and its "fine adjustment" is set to the "calibration" position. Then its cycle 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. Lissajous figure method to measure frequency

Set the oscilloscope to XY working mode, input the measured signal into the Y axis, and the standard frequency signal into "X external", and slowly change the standard frequency until the two signal frequencies become integer multiples, for example, 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 phase of the two deflection voltages, but also to the frequency of the two deflection voltages. The tracing method can be used to draw Lissajous figures at various frequency ratios of ux and uy and different phase differences. Several Lissajous figures with different frequency ratios are shown in Figure 5-15.

By using the relationship between Lissajous figures and frequency, accurate frequency comparison can be performed to determine the frequency of the measured signal. The method is to draw horizontal and vertical lines through the Lissajous figure respectively, and the drawn horizontal and vertical lines should not pass through the intersection of the figure or be tangent to it. If the number of intersections between the horizontal line and the figure is m, and the number of intersections between the vertical line and the figure is n, then

fy / fx = m / n

When the standard frequency fx (or fy) is known, the frequency fy (or fx) of the signal under test can be obtained from the above formula. Obviously, in actual testing work, when using Lissajous figures 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 to make the figure displayed on the screen a circle or ellipse. At this time, the frequency of the signal under test is equal to the frequency of the known signal.

Since the two voltages applied to the oscilloscope have different phases, the graph on the screen will have different shapes, but this has no effect on the determination of the unknown frequency.

The Lissajous figure method is quite accurate in measuring frequency, but it is time-consuming to operate. At the same time, it is only applicable to measuring signals with lower frequencies.