In recent years the cathode-ray tube (crt) has become familiar to millions of persons as the picture tube in a television set. The crt in television is designed to reproduce an undistorted picture on a screen.
The picture is developed from a series of pulses and varying voltages applied to the elements of the tube. Fundamentally, the crt consists of an electron "gun," a phosphorescent screen, and deflecting devices to control the movement of the electron beam "shot" from the gun (see Fig. 12.47).
Cathode Ray Tube Diagram
How Cathode Ray Tube Works?
As in any thermo-emitting tube, the heated cathode supplies the electron emission, and these electrons are accelerated toward the screen by the positive charges on the anodes. The intensity of the electron beam is regulated by means of the control grid charge.
After the electron beam is accelerated and focused by the anodes, it is controlIed in direction by the deflection plates. When the electrons strike the phosphorcoated screen, they cause a bright spot to appear.
If an alternating voltage is applied to the vertical deflection plates, the spot will move up and down and form a straight line. In like manner, if an alternating voltage is applied to the horizontal deflection plates, a horizontal straight line will appear on the screen.
In practice the horizontal deflection of the electron beam is used to provide a time base. The output of a sawtooth oscillator is applied to the horizontal deflection plates so that the electron beam will sweep at a steady rate from left to right and at the end of the sweep will return instantly to the left side and start another sweep.
As the voltage rises, the electron beam moves to the right; but when the voltage drops, the beam returns immediately to the left side of the screen.
If we set the horizontal timing for the crt to 60 Hz and apply a 60-Hz alternating voltage to the vertical deflection plates, a stationary sine wave will appear on the screen.
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