bias circuit, horizontal and vertical deflection coils,
and CRT. Symbols are drawn at the rate of 50 times a
second on the CRT. The rapidly moving electron
beam is generated by the bright-up pulse from the
signal data processor.
The bright-up pulse is
amplified by the bright-up amplifier, and then applied
to the CRT grid. The CRT, which is normally biased
into cutoff, is turned on, and an electron beam is
emitted. The electron beam strikes the CRT face at a
position determined by the amount of current flowing
in the horizontal and vertical deflection coils. The
amount of light (symbol brightness) emitted from the
CRT is controlled by the cathode bias circuit. A
control voltage from the control panel is applied to the
cathode bias circuit, which serves as a reference
voltage for the autobrilliance sensor and the CRT
cathode. The voltage output from the autobrilliance
sensor is amplified by the autobrilliance amplifier.
The output is applied to the cathode bias circuit,
which changes the amount of bias (brightness) on the
CRT. Thus, an optimum CRT contrast is constantly
maintained under varying ambient light.
DEFLECTION MODULE.— The deflection
module contains the X and Y deflection amplifiers, X
and Y low amplifiers, X and Y comparators, and one
OR gate. The type and location of each symbol on the
combiner is determined by X and Y analog voltages
applied to the deflection circuits. As the need for a
symbol arises, the busy signal from the HUD is
sampled by the symbol generator. The busy signal is
generated any time current is flowing in either
deflection coil. Comparators, connected across each
coil, are used to detect when the deflection coil
current is equal to zero. As the current in either
deflection coil reaches zero, the output from the
comparator, connected across the coil, goes low.
When both comparator outputs are low, the OR gate
output goes low, and the busy signal is removed from
the symbol generator input. At the same instant, the
symbol generator applies a precise analog voltage to
the X and Y deflection amplifiers. The output from
the deflection amplifiers causes current to flow in the
deflection coils.
The current amplitude is
precalculated to drive the CRT electron beam to the
start point of the symbol to be drawn. A bright-up
pulse is then applied to the bright-up amplifier in the
video module. At the same instant, the deflection
voltage is modulated at a predetermined rate and
amplitude by the symbol generator. The modulation
is detected by the deflection amplifiers and applied to
each deflection coil. A specific part (or all) of a
symbol is then drawn on the CRT face in a precise
location. The bright-up pulse continues long enough
to draw the prescribed line or circle that makes up the
symbol. When the bright-up pulse is removed, the
CRT is driven into cutoff until the next bright-up
pulse is applied. This process is repeated until all
symbols have been displayed on the CRT.
BITE Functions
Both the signal data processor and the heads-up
display unit contain built-in test equipment (BITE).
SIGNAL DATA PROCESSOR.— The built-in
test equipment contained in the signal data processor
consists of a clock pulse monitor, data test equipment,
raw data test equipment, six low-voltage comparators,
a signal data processor fail indicator, and a thermal
overload sensor.
HEADS-UP DISPLAY UNIT.— The built-in test
equipment contained in the HUD unit consists of a
high-voltage comparator, seven low-voltage
comparators, a bright-up parity circuit, a HUD fail
indicator, and a thermal overload sensor.
SYMBOLOGY
The symbols that are used with the various modes
of operation in the F-14 are listed and described in
figure 5-9. These symbols give the pilot important
information, such as aircraft attitude, heading,
altitude, angle-of-attack, and ground track during
flight. Attack information, such as closure rate, range
to target, maximum range for weapon launch,
minimum range for weapon launch, and boresight
reference, is available during air-to-air and
air-to-ground operations.
Ordnance information,
such as number of rounds remaining, the type of
weapon selected, and the number of weapons ready
for launch, is also available.
DECLUTTER
The system used in the F-14 (AVA-12) has a
feature known as declutter. It is used to remove
preselected, unwanted symbols from the display
during certain modes of operation. This feature is
especially important during an air-to-air situation.
When the pilot is trying to locate and engage an
enemy target, the declutter feature will clear the
unnecessary symbols from his/her view.
5-10
