4-32 888-2408-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
The following is a listing of all of the Default Values which are stored in the
Firmware:
• Forward Factor - 0.007697
• Reflect Factor - 0.000306
• Cal APC Factor - 7.58
• Cal Frequency - 98MHz
• Operating Frequency - 98MHz
• IPA Factor - 200
• Exciter Factor - 200
• TPO 100% - 05.00kW
• Max High - 06.00kW
• Max Low - 2.50kW
• UPS Setting - 2.50kW
• Low Power Alarm - 4.50kW
• VSWR Foldback - 1.35
• VSWR Fault - 1.5
4.7.1.2 APC, Automatic Power Control
One important function of the Master Controller is transmitter power control, which
is accomplished via the APC Loop. The APC Loop controls the transmitter power
and keeps it constant by varying the IPA RF output indirectly proportionate with the
forward power. In other words, if the forward power output goes up, the IPA output
will be reduced, which will in turn reduce the PA RF output. If the forward power
output goes down, the IPA RF output will be increased, which will increase the PA
RF output to compensate.
The APC loop starts at the forward power directional coupler. The forward sample
is connected to the Life Support Board where it is calibrated with R127 for 8.9V@
100% TPO at TP9. This voltage sample is then sent to the Master Controller
(schematic sheet 7) where it is squared by multiplier chip U1 (to make the voltage
from the coupler proportional to power) and divided by 10. This is then sent to U5-
6 where it is compared to APC_REF voltage (APC Reference).
APC_REF is the DC output of a D/A Converter or DAC whose input is called
DAC_APC_REF and is controlled by the RAISE and LOWER buttons on the front
of the controller (or by the remote control inputs) or can be varied automatically by
the Master Controller’s foldback routines. Pressing RAISE increases the APC_REF
voltage and pressing LOWER decreases it. The DAC_APC_REF is responsible for
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