Q. Explain the use of CRO for frequency measurement.

Measurement of frequency: The frequency of the periodic signals is easily measured with a CRO. The frequency of a signal to be measured is compared with a known frequency. A sinusoidal voltage of unknown frequency (the frequency which is to be measured) is applied to the vertical plates. If the ratio of two frequencies (f and f) is an integer, a clearly defined and distinctive pattern appears on the screen. These patterns are known as Lissajous pattern.

Let us assume that two sinusoidal voltages applied to the vertical and horizontal plates of the CRO have equal frequency and same phase and let that voltages applied to the plates be given as voltage at vertical plate.

When the frequency of the two voltages is not equal then to obtain the Lissajous pattern is not simple. Lissajous pattern may be classified into two general categories, the closed and the open Lissajous patterns. A closed Lissajous pattern is continuous and has no break it has no free ends as shown in fig. The open patterns are simply steady traces and have free ends. 

The ratio of vertical to horizontal frequencies of closed Lissajous patterns can be obtained by counting the number of positive vertical peaks and dividing it by number of positive horizontal peaks and is given by the formulae.

Lissajous patterns are used for accurate measurement of frequency of a waveform. The signal or the waveform, whose frequency is to be measured, is applied to the vertical deflection plates the CRO, an accurately calibrated standard frequency source is used to supply voltage to the horizontal deflection plate with the internal sweep generator switched off.