CRO and its Working

Hi friends, in this post we are going to learn about Basics of Cathode Ray Tube (CRO) and its function. We will also see the working of CRO.

Block Diagram of CRO:

CRO block diagram
CRO block diagram

Cathode Ray Tube (CRT):

CRT Produces a sharply focused beam of electrons, accelerated to a very high velocity. This electron beam travels from electron gun to the screen. The electron gun consists of filament, cathode, control grid, accelerating anodes and focusing anode. While travelling to the screen, electron beams passes between a set of vertical deflecting plates and a set of horizontal deflection plates. Voltages applied to these plates can move the beam in vertical and horizontal plane respectively. The electron beam then strikes the fluorescent material (phosphor) deposited on the screen with sufficient energy to cause the screen to light up in a small spot.

Vertical Amplifier:

The input signal is applied to vertical amplifier. The gain of this amplifier can be controlled by VOLT/DIV knob. Output of this amplifier is applied to the delay line.

Delay Line:

The delay Line retards the arrival of the input waveform at the vertical deflection plates until the trigger and time base circuits start the sweep of the beam. The delay line produces a delay of 0.25 microsecond so that the leading edge of the input waveform can be viewed even though it was used to trigger the sweep.

Trigger (Sync.) Circuit:

A sample of the input waveform is fed to a trigger circuit which produces a trigger pulse at some selected point on the input waveform. This trigger pulse is used to start the time base generator which then starts the horizontal sweep of CRT spot from left hand side of the screen.

Time base (Sweep) Generator:

This produces a saw – tooth waveform that is used as horizontal deflection voltage of CRT. The rate of rise of positive going part of sawtooth waveform is controlled by TIME/DIV knob. The sawtooth voltage is fed to the horizontal amplifier if the switch is in INTERNAL position. If the switch is in EXT. position, an external horizontal input can be applied to the horizontal amplifier.

Horizontal Amplifier:

This amplifies the saw – tooth voltage. As it includes a phase inverter two outputs are produced. Positive going sawtooth and negative going sawtooth are applied to right – hand and left – hand horizontal deflection plates of CRT.

Blanking Circuit:

The blanking circuit is necessary to eliminate the retrace that would occur when the spot on CRT screen moves from right side to left side” This retrace can cause confusion if it is not eliminate. The blanking voltage is produced by sweep generator. Hence a high negative voltage is applied to the control grid during retrace period or a high positive voltage is applied to cathode in CRT.

When a sawtooth voltage is applied to horizontal plates and an input signal is applied to vertical plates, display of vertical input signal is obtained on the screen as a function of time.

Power Supply:

A high voltage section is used to operate CRT and a low voltage section is used to supply electronic circuit of the oscilloscope.

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Tags: Working of CRO, How CRO generates waveforms, Working principle of CRO.

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