cathode  ray oscilloscope (CRO) The cathode ray oscilloscope is an  electronic instrument  used  mainly  for  (i)  studying  the waveform  of  an  ac  signal,  sound  or even  heartbeats,  (ii) measurements  of ac voltages, and (iii) comparison of frequencies of two ac signals.

It  consists of a cathode ray tube and associated electronic circuits.  The  ac  voltage  is applied  to the Y plates of the cathode ray tube* . The spot on the screen will trace out a vertical line proportional to the applied voltage. In order to convert this into a waveform, a sweep  voltage  is applied into the X plates, by a circuit called time  base. When  sweep  cycle  is  synchronized  to the signal frequency,  the  image  is  repeated on  the  screen and appears stationary (see fig. c3).

To  find  the  frequency  of ac voltage measure the distance between  the  peaks, D x (see fig.c4). Note the setting of the time  base control (t ms/cm). Time between peaks (T) is the product  of D x and t. Reciprocal of T gives the frequency of ac voltage.

Measure  the  amplitude of the waveform (that is. center line to the  peak)  on  the screen (y cm). Also note the setting of the vertical  gain  control  ( m V/cm). The peak voltage is m ´ y volts.

If  two  ac  signals  are fed simultaneously, one into the X plates of unknown frequency and the other into the Y plates (from an  oscillator),  Lissajous  pattern appears on the screen that depend   on  the  ratio of two frequencies (fig. c5). By varying the frequency  of  the  oscillator, we can get desired shape and find the unknown frequency.