Cathode Ray Oscilloscope CRO: Its Principle, Construction, Working of a Cathode Ray Oscilloscope and its uses

It is very versatile electronic apparatus/device. It is in fact a high speed/rapid graph plotting device. It displays the  appearance and measurement (shape or size) of an electrical signal on the screen.It is provides visual images of varying electrical quantities. Also, it provides accurate time and amplitude measurements of voltage signals over a wide range of frequencies (rhythm).


When a charged particle moves in an electric field, a force acts on it.

For Your Information


Potential at cathode is -1000V Potential at grid is -1050V Potential at focusing anode is -6000V. Potential at accelerating anode is 0V.


A  CRO (cathode ray oscilloscope) consists of:

i) Electron gun ii) Deflection system iii) Display system

i) Electron Gun

The electron gun produces a beam of electrons. It consists of;

1) Filament

When current is passed through the filament, it glows and heat from it heats up the cathode.

2) Cathode

When cathode is heated, it emits electrons by thermionic emission.

3) Grid

It is at negative potential relative to cathode. It controls the number of electrons e⁻ (negitively charged particles) reaching the screen and thus controls/manage the brightness of spot on the screen.

4) Focusing Anode

it' is at positive potential relative to cathode. The electrons that pass through this anode are focused into a fine beam.

5) Accelerating anode

It is at zero potential. Its main function is to accelerate the electrons.

ii) Deflection system

The beam of electrons from anodes firstly passes through a pair of horizontal deflecting plates called X-plates which deflect the beam horizontally on the screen i.e., parallel to x-axis and then between two vertical plates called Y- plates which deflect the beam vertically on the screen i.e., along the y-axis.

iii) Display System

The screen of CRO is coated with a fluorescent material (zinc sulphide). The zinc sulfide gives a glow of light when electrons collide with it.

Working of CRO

Voltage across X-plates

The voltage that is applied across the x-plates is usually provided by a circuit that is built in CRO (known as time base generator).

Time base generator

Its output wave form is saw-tooth. The voltage increases linearly with time for period T and then drop to zero. As this voltage is applied across X-plates, the spot moves along x-axis for a time T. As after time T, the saw tooth voltage becomes zero, So the spot rapidly reaches to its initial position at the end of each period T. If the time period is very small, the spot moves so quickly that we see just a bright line on the screen.

Voltage across Y-plates

If a sinusoidal voltage is applied across y-plates with time base generator off, the spot moves vertically up and down at the frequency of applied AC voltage. This produces a vertical straight line on the screen whose length represents the peak to peak value of applied voltage! If a sinusoidal voltage is applied across y-plates time with base generator ON, the vertical straight line now spread out and appears as sinusoidal trace on the screen.

The pattern will appear stationary only if the time T is equal to or is some multiple of the time of one cycle of the voltage on y plates.

It is thus necessary to synchronize the frequency of the time base generator with the frequency of the voltage at they plates. This is possible by adjusting the synchronization controls provided on the front panel of the CRO.

Uses of CRO

CRO is used:

a) display the waveform of a given voltage.

b) measure the voltage, frequency and phase of the input signal.

c) measure the instantaneous value, peak value.

d) measure the time period of AC signals.

e) measure the phase difference between two voltages as shown in figure.

No comments