Measuring frequencies and phases

If alternating sinusoidal voltages with the same frequencies are applied to both pairs of deflecting plates of the oscilloscope, without using a sweep (Fig. 1), a still image (figure) is obtained on the screen, the form of which depends on the ratio of frequencies and phases.

Fig 1. Scheme for measuring the frequency and phase shift with an oscilloscope

If two voltages coincide in phase, then the movement of the beam horizontally and vertically occurs simultaneously and proportionally, therefore the trace on the screen is obtained in the form of a straight line sloping upward from left to right. If they are not in phase, then an ellipse appears on the screen (Fig. 2).

Fig 2. The image on the oscilloscope screen at different phase shifts between the voltages on the horizontal and vertical plates

With the coincidence or opposite of the phases, straight lines are obtained, with other phase values ​​- ellipses.

If the stresses are selected in such a way that the maximum deviations on both axes are the same, then with a phase difference of 90 ", circles will be obtained. However, straight lines and circles are special cases of a general elliptical curve.

To estimate the voltage frequency, it is applied to one pair of plates, and a reference voltage to the other pair (Fig. 3).

Fig 3. Figure on the oscilloscope screen with a multiple ratio of frequencies on plates X and Y

If the frequency of the voltage under investigation is a multiple of the frequency of the reference voltage and there is a numerical ratio between them (for example, 3: 2), then a fixed curve will appear on the screen. The frequency ratio can then be determined from the shape of the resulting curve, called the "Lissajous figure".

O. Vershinin I. Mironenko

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