To study a.c. wave forms using a cathode ray oscilloscope
In addition to the tube, a cathode ray oscilloscope contains a sweep generator or time-base circuit. When this is switched on it applies a potential difference to the Xplates which builds up uniformly with time to a maximum and then repeats the process at regular intervals. The result is that the spot moves horizontally across the screen with steady velocity, returns to zero instantaneously (fly-back) and repeats the cycle continuously. Thus at low sweep frequencies the trace appears as a moving spot but, owing to persistence of vision it becomes a continuous line at higher frequencies.
If a signal in the form of a simple a.c. voltage is applied across the Y-plates, the spot will oscillate up and down in time with the voltage. The resultant trace on the screen will be a combination of the Y-plate a.c. signal and the X-plate sweep. In effect, the moving spot plots on the screen a luminous graph of a.c. voltage variation with time (see Fig. 44.14).
Obviously, a single complete oscillation or wave will be observed when the sweep frequency is equal to the a.c. signal frequency. Otherwise two or more complete waves will be seen when the signal frequency is twice or some integral (wholenumber) multiple of the sweep frequency.
Using a cathode ray oscilloscope the complex wave forms from an amplifier fed by a microphone or record player may be examined.