Measurement of Phase using CRO

Measurement of Phase using CRO

Cathode ray oscilloscope is used to determine the phase difference between two signals of the same frequency. The procedure is to apply one of the signals to the horizontal deflecting plates and the other to the vertical deflecting plates. The result is a pattern that reflects the character of the phase difference. Typical pattern for a phase angle is shown below in Figure.

The phase difference q between the two signals is given by

Sin θ = ± B/A

Where A is the maximum vertical deflection produced and B is Y intercept.

Irrespective of the relative amplitudes of the applied signal voltages the ellipse provides a simple means of finding the phase difference between the signals. The gains of vertical and horizontal amplifiers can be adjusted to get the ellipse in a square marked by the coordinate lines on the graticule. If the major axis is in the first and third quadrants the phase angle is between 0" and 90° or 270° and 360°. If the major axis passes through the second and fourth quadrants the phase angle is between 90° and 100° or between 180° and 270°. 

Measurement of Time Interval using CRO: 

Time intervals of short duration can be measured using CRO. The sweep is triggered by the pulse that serves as a time reference. The length or duration of this driven sweep is adjusted so that both the pulses are visible on the screen. If the approximate sweep speed is known, a rough measurement of the time interval separating the two pulses can be determined from the linear distance separating the pulses on the screen.

Greater accuracy can be had by markers (or indexes or cursor). Makers can be superimposed upon the cathode ray trace. These markers have their origin in a sine wave oscillator of accurately known frequency. The waveform from this oscillator is then modified by amplification, clamping and differentiating until a series of sharp pulses are obtained.

The operator or user has to adjust the position of the marker until it coincides with the observed pulse, the time interval is given by the setting of the position control.