Cathode Ray Oscilloscope

Cathode Ray Oscilloscope:

A cathode ray oscilloscope is an instrument used for displaying the waveform of a given electrical signal. It is called so because it traces the waveform with the help of a CR tube.

Construction- A CRO consists of a CR tube together with specially designed circuits. The CR tube has three components, namely

Electron Gun.
Deflecting Section.
Fluorescent Screen.

Electron Gun- Electrons are emitted by a hot filament F, F. The number of electrons in the electron beam is regulated by grid G and the beam is focussed by applying suitable voltages across the anodes A₁ and A₂ and A₃.

Deflecting Section- In this section one each pair of horizontal and vertical plates is placed. By applying suitable voltages across the XX plates, the beam may be deflected in the horizontal line. Similarly, by applying voltages across the YY plates, the beam may be deflected vertically.

Fluorescent Screen- The electron beam strikes and excites a phosphor screen. Thus, a spot of light appears on the screen.

Working-The X, X plates are given a steadily rising voltage which after reaching a certain maximum value falls rapidly to zero. This process is repeated at regular intervals as shown below. Such a voltage is called a saw-tooth voltage. The frequency of a saw-tooth voltage can be adjusted within wide limits.

Under the action of the saw-tooth voltage, the spot is swept steadily towards the right and then it flies back swiftly and starts all over again. A horizontal line is, therefore, traced on the screen of the CRO. This arrangement of getting a horizontal line with the help of saw-tooth voltage is called a time base.

Now if a time-dependent potential difference is simultaneously applied across the YY plates of the CRO, the spot of light moves up and down due to the applied electrical signal as well as it also moves from left towards right due to saw-tooth voltage applied across the X, X plates. The spot, therefore, traces the waveform of the given signal.

Applications: The CRO is used to investigate the waveform of a given signal. A modified CRO tube is used in TV receivers for tracing the picture corresponding to the signals transmitted by the transmitter.