
30
|
ni.com
|
NI Digital System Development Board User Manual
Figure 10.
Color CRT Display
Electron beams emanate from
electron guns
which are finely-pointed heated cathodes placed in
close proximity to a positively charged annular plate called a
grid
. The electrostatic force
imposed by the grid pulls rays of energized electrons from the cathodes, and those rays are fed
by the current that flows into the cathodes. These particle rays are initially accelerated towards
the grid, but they soon fall under the influence of the much larger electrostatic force that results
from the entire phosphor-coated display surface of the CRT being charged to 20 kV (or more).
The rays are focused to a fine beam as they pass through the center of the grids, and then they
accelerate to impact on the phosphor-coated display surface. The phosphor surface glows
brightly at the impact point, and it continues to glow for several hundred microseconds after the
beam is removed. The larger the current fed into the cathode, the brighter the phosphor will glow.
Between the grid and the display surface, the beam passes through the neck of the CRT where
two coils of wire produce orthogonal electromagnetic fields. Because cathode rays are composed
of charged particles (electrons), they can be deflected by these magnetic fields. Current
waveforms are passed through the coils to produce magnetic fields that interact with the cathode
rays and cause them to transverse the display surface in a
raster
pattern, horizontally from left
to right and vertically from top to bottom, as shown in Figure 11. As the cathode ray moves over
the surface of the display, the current sent to the electron guns can be increased or decreased to
change the brightness of the display at the cathode ray impact point.
Information is only displayed when the beam is moving in the
forward
direction (left to right and
top to bottom), and not during the time the beam is reset back to the left or top edge of the display.
Much of the potential display time is therefore lost in
blanking
periods when the beam is reset
and stabilized to begin a new horizontal or vertical display pass. The size of the beams, the
frequency at which the beam can be traced across the display, and the frequency at which the
electron beam can be modulated determine the display resolution.