CRT Principles

The primary component in a display monitor is the display tube or cathode ray tube (CRT). This fanshaped glass tube contains a vacuum through which electrons may travel. At the rear end of the CRT is a (usually nickel) cylindrical cathode, or electron gun, which is heated by a small 6.3 V heater in order to “boil off” electrons - this is the principle of thermionic emission.

Electrons, being negatively charged, are attracted to the positively charged anode at the other end of the tube, and accelerate, in a stream, to strike the phosphor coating on the inside of the glass face of the tube.When the electron beam hits the phosphor, the phosphor emits light; this light can be green, orange, pale blue or white, depending on the type of phosphor, and the glow might persist for some time or fade quickly.

The anode has a very high positive voltage applied - typically tens of thousands of volts - and it can be easily identified by the large rubber cap over its connector behind the tube face, and the thick rubber insulation on its wiring.

Left to its own devices, the electron stream will simply strike a single point in the centre of the tube face.However, because amagnetic field exerts a force on charged particles we can apply a magnetic field around the electron beam, and “bend” the stream to strike any spot on the face of the tube.

The magnetic field is supplied by two sets of electromagnets mounted on a yoke and wrapped around the neck of the tube; one set deflects the beam horizontally while the other bends the beamin the vertical axis. On many CRTs you may also find a set of coils behind the deflection yokewhich is used to focus the electron beam.

By applying a sawtooth waveform to the horizontal deflection coils we can cause the electron beam to repeatedly move slowly from left to right across the screen and then quickly “fly back” to the left hand side again. Likewise, by applying a lower frequency sawtooth to the vertical deflection coils, we can cause the beam to slowly move down the screen and quickly return to the top.

Incidentally, the high frequency used for the horizontal scan is ideal for driving a transformer (called a flyback transformer) which steps it up to the high voltage required for the CRT anode. You can find the flyback transformer by following the thick insulated wire back from the anode - but keep your fingers clear!

In the case of a computer monitor (as opposed to a television screen), the electron beam is normally turned off, and is turned on when we want to illuminate a particular spot on the screen. This is known as modulation. By doing this under the control of a character generator as part of anadapter cardwe can display text on the screen or, by writing directly into videomemory,wecan create graphics.

The electron beam is modulated only during the horizontal forward scans, and is turned off during both the horizontal and vertical retrace intervals. But this introduces another complication: somehow the information coming from the graphics adapter must be synchronised to the sweeps of the electron beam (or vice versa,which is actually the case).

This is achieved by the generation, by the graphics adapter, of horizontal and vertical sync pulses, each ofwhich triggers the retrace and positions the electron beam at the left or top of the screen. Some video cards provide separate horizontal and vertical sync signals, others combine them into a composite sync signal, while a few (like CGA) have a composite video output in which the syncs and video are all combined into a single signal (usually for use with domestic television sets or television monitors rather than computer monitors).

By David Stott