Cathode-Ray Tubes

Many TV receivers, and some desktop computer monitors, use cathode-ray tubes (CRTs). So do older oscilloscopes, spectrum analyzers, and radar sets.

Electron Beam

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Simplified schematic diagram of a grounded-grid RF amplifier circuit using a triode tube.
In a CRT, a specialized cathode called an electron gun emits an electron beam that is focused and accelerated as it passes through positively charged anodes. The beam then strikes a glass screen whose inner surface is coated with phosphor. The phosphor glows visibly, as seen from the face of the CRT, because of the effect of the high-speed electrons striking it.
 
The beam scanning pattern is controlled by magnetic or electrostatic fields. One field causes the beam to scan rapidly across the screen in a horizontal direction. Another field moves the beam vertically. When complex waveforms are applied to the electrodes that produce the deflection of the electron beam, a display pattern results. This pattern can be the graph of a signal wave, a fixed image, an animated image, a computer text display, or any other type of visible image.

Electromagnetic CRT

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Simplified cross-sectional rendition of an electromagnetic CRT.
A simplified cross-sectional drawing of an electromagnetic CRT is shown in above figure. There are two sets of deflecting coils, one for the horizontal plane and the other for the vertical plane. (To keep the illustration reasonably clear, only one set of deflecting coils is shown.) The greater the current in the coils, the greater the intensity of the magnetic field, and the more the electron beam is deflected. The electron beam is bent at right angles to the magnetic lines of flux.
 
In an oscilloscope, the horizontal deflecting coils receive a sawtooth waveform. This causes the beam to scan, or sweep, at a precise, adjustable speed across the screen from left to right as viewed from in front. After each timed left-to-right sweep, the beam returns, almost instantly, to the left side of the screen for the next sweep. The vertical deflecting coils receive the waveform to be analyzed. This waveform makes the electron beam move up and down. The combination of vertical and horizontal beam motion produces a display of the input waveform as a function of time.

Electrostatic CRT

In an electrostatic CRT, charged metal plates, rather than current-carrying coils, are used to deflect the electron beam. When voltages appear on these deflecting plates, the beam is bent in the direction of the electric lines of flux. The greater the voltage applied to a deflecting plate, the stronger the electric field, and the greater the extent to which the beam is deflected.
 
The principal advantage of an electrostatic CRT is the fact that it generates a far less intense magnetic field than an electromagnetic CRT. This so-called extremely low frequency (ELF) energy is a cause for concern, because it might have adverse effects on people who use CRT-equipped devices, such as desktop computers, for extended periods of time. In recent years, with the evolution of liquid crystal displays (LCDs) and plasma displays as alternatives to the CRT type of display, ELF has become a much less significant concern.