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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