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What is CRO or Cathode Ray Oscilloscope?

by Harpreet_Physics

CRO or Cathode Ray Oscilloscope

Cathode Ray Oscilloscope is one of the most important electronic measuring instruments. CRO or Cathode Ray Oscilloscope displays and measures the electrical signals, time intervals and phase shift between two electrical signals. 

Non-electrical quantities such as temperature, pressure and strain measured by first converting them into an equivalent voltage using an appropriate transducer.

Any CRO or Cathode Ray Oscilloscope consists of the following seven major sections

i)   Cathode Ray Tube (CRT)

ii)   Timebase circuits

iii)   Trigger circuits

iv)   Vertical Circuits

v)   Horizontal Circuits

vi)   High Voltage Power Supply

vii)  Low Voltage Power Supply

The arrangement of these sections in a CRO is shown in fig in the form of a Block diagram.

Block diagram of CRO or Cathode Ray Oscilloscope
Figure : Block diagram of CRO or Cathode Ray Oscilloscope


A Cathode Ray Tube (CRT) is a vacuum tube. An electron beam in CRT controlled by electric or magnetic fields. It generates a visual display of input electrical signals on a fluorescent screen. It consists of three important parts, an electron gun, deflection system and a fluorescent screen.

In the CRT, the electron gun generates an electron beam, focuses it, and accelerates it towards a fluorescent screen located at the further end of the tube. The electron beam may be moved to any spot on the screen with the help of a deflection system. A CRT with electrostatic deflection forms the central and main part of the CRO. 


Display of the signal variation by the electron beam needs the beam to move horizontally at a uniform rate across the screen. Ramp voltage or saw-tooth voltage satisfies this condition . The ramp voltage is generated by the Time Base Circuit.

The time base circuit consists of a time base generator. The time base generator is a variable frequency oscillator that produces an output voltage of saw-tooth shape.


Synchronization is the method of adjusting the frequency of the time base generator to the frequency of the input signal. On the CRO screen a stationary wave pattern seen.

The signal will be properly synchronized only when its frequency equals the sweep frequency or sub-multiple of sweep frequency. 

One of the methods of achieving synchronization is the use of a trigger circuit. The trigger circuit initiates the time base so that the horizontal deflection sweeps in synchronization with the vertical signal. 


An attenuator and a voltage amplifier  are major parts of  vertical circuits. The signal fed at the Y-input. Then It reaches the attenuator input. By changing the amount of attenuation, the signal amplitude is increased or decreased. Then it is fed to the voltage amplifier’s input. As a result adequate deflection obtained on the screen.


The sweep generator output cannot drive the horizontal plates directly. Therefore it must be initially amplified. Voltage amplifier is the major part of the horizontal circuit. 


The low voltage power supply powers the electronic circuits such as amplifiers, time base generator, trigger circuit. An output of the order of few tens to a few hundreds of volts obtained.


The high voltage power supply provides voltages to anodes in the electron gun assembly. It supplies voltages of the order of 1600V to 2200V.

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