one channel increases, the rate of change in the other
decreases. If the circle is large, the rate of change in
the analog defection voltage output is large. If the
circle is small, the analog voltage will be small. The
rate of change is inversely proportional to the contents
of the parameter register, which contains the inverse
of the circle radius. This causes the generator to
produce a circle with the desired radius.
The bright-up pulse control is from a quadrant
counter in the input/output buffer and control. This
turns on the bright-up pulse after the electron beam
travels halfway around the circle. This delay
compensates for the slow response time of the
deflection circuits in the HUD. The bright-up pulse
stays on until the quadrant counter counts four times.
A short time after the fourth count, the bright-up pulse
turns off, and the symbol generator busy signal
terminates. This completes the circle mode.
Analog-to-Digital Conversion (ADC) Mode.
The third mode of symbol generator operation is the
analog-to-digital conversion mode. In this mode, the
operation of both the X and the Y channels are the
same. Therefore, only the X channel operation is
described. At the start of the ADC mode, the most
significant bit in the parameter register becomes
preset to the one level.
The rate and deflection registers are preset to zero
and coupled directly to each other.
coupling of the two registers provides for duplication
of the contents of either register.
The third step gates a specific analog voltage
from the X channel input selector to one side of the X
The fourth step gates the digital-to-analog
converter (DAC) voltage from the X channel DAC to
the other side of the X channel comparator. The DAC
voltage represents the contents of the X channel
If a difference in voltage level
exists between the two applied voltages, the
comparator has an output applied to the deflection
register. The output of the comparator represents the
sign of the difference. This output causes the contents
of the parameter register to add to or subtract from the
contents of the rate register.
On the next operation, the one in the parameter
register shifts to the next lower bit location, and the
voltage comparison phase repeats. This process
continues until the parameter register reaches zero.
When at zero, the contents of the rate register (in
digital form) are equal to the input selector analog
voltage at the comparator. The next operation
transfers the contents of the rate register to memory
by way of the data feedback (DFB) lines. In later
operations, the remaining analog voltages go to the
comparator, and the entire process repeats. The ADC
mode continues until all analog voltages are
converted to digital data and stored in memory.
Heads-Up Display Unit
Aircraft ac power applied to the low-voltage
power supply is rectified, and then divided into seven
different dc levels for distribution throughout the
display. A transistorized oscillator in the high-voltage
power supply receives 24 volts of dc excitation from
the low-voltage power supply. The oscillator output
is applied to a voltage multiplier and rectifier circuit,
which increases the input voltage to 15,000 volts.
The 15,000 volts are applied to the CRT anode and a
voltage divider within the high-voltage power supply.
Outputs from the voltage divider are used to control
the oscillator frequency and a comparator output. The
comparator senses any differences that may exist
between the voltage divider output and a fixed
reference voltage. If the high voltage drops below a
prescribed level, a fail signal is generated by the
comparator and applied to the BITE circuit.
OPTICAL MODULE. The optical module
contains the standby reticle, in-range indicator,
control panel, autobrilliance sensor, and lens unit.
The standby reticle is used only when the HUD is
inoperative, and is controlled electrically and
mechanically from the control panel. Electrical
power used for the operation of the standby reticle is
obtained from outside the HUD.
Light emitted from the standby reticle is
transmitted through the lens unit to the combiner. The
in-range indicator indicates when a range discrete
signal is received from the forward-looking radar set.
Symbols are represented by light emitted from a CRT
located in the video module. As symbols are drawn
on the CRT face, the emitted light is received by the
lens unit and applied to the combiner. The desired
level of brightness is selected from the control panel
by adjustment of the voltage level applied to the
cathode bias circuit. The autobrilliance sensor is used
to detect ambient light changes. Any change in
ambient light proportionally changes the output
voltage level of the autobrilliance sensor.
VIDEO MODULE. The video module contains
the bright-up and autobrilliance amplifiers, cathode