VHF 100 W Power Amplifier Module
68P81096E24-C 3
06/30/06
The following theory of operation describes the operation of the PA circuitry at a functional level. The informa-
tion is presented to give the service technician a basic understanding of the functions performed by the module
in order to facilitate maintenance and troubleshooting to the module level. Refer to Figure 2 for the following
functional theory of operation.
RF Signal Path
A low-level modulated RF signal (approximately +12 dBm) from the Excit-
er Module enters the PA Input board via a coax cable. The signal is input
to the Final Module and amplified up to 135 W (depending on the dc power
control voltage (V_CNTL) from the PA output board). The output of the Fi-
nal Module is fed to the circulator.
The output of the circulator exits from the PA Output board via an N-type
coax connector.
Circulator
A single circulator is built into the Power Amplifier Module to provide iso-
lation between the PA module and the transmit antenna.
Heat Sink Temperature Sense
A thermistor mounted on the PA printed circuit board outputs a variable
resistance proportional to the heat sink temperature. This signal is fed to
the power control circuitry on the PA output board, which monitors the sig-
nal and reduces the PA output power (and enables a fan) if the PA temper-
ature exceeds set limits.
Output Power Control
A feedback and control loop configuration is used to regulate the PA output
power. The Directional Coupler generates a dc voltage proportional to the
PA Module output power, which is a maximum of 100 W. This voltage is
fed to power control circuitry on the PA output board where it is compared
to reference voltages to generate a dc power control voltage. The Station
Control Module sends current and power limit signals to a D/A converter
(via the SPI bus) to control the maximum current/control voltage levels.
The control voltage is fed to the Final Module where it controls the Final
Module output, thus controlling the overall output from the PA Module.
3 FUNCTIONAL THEORY OF OPERATION
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