Motorola MTR2000 T5544 Instruction / Field Service Manual page 171

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FUNCTIONAL THEORY OF OPERATION
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
Circulator
Heat Sink Temperature Sense
Output Power Control
68P81096E26-B
09/30/05
A low-level modulated RF signal (approximately +13 dBm) from the Excit-
er Module enters the PA Input board via a coax cable. The signal is input
to the IPA and amplified to approximately 0 to 10 W (depending on the dc
power control voltage (V_CNTL) from the PA output board). The IPA out-
put is then fed to the Butterfly Module where final amplification occurs.
The output of the Butterfly (40 W maximum) exits from the PA Output
board via an N-type coax connector.
A double circulator is built into the CLN1227 Power Amplifier Module to
provide isolation between the PA module and the transmit antenna.
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 if the PA temperature exceeds set lim-
its.
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. 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 max-
imum current/control voltage levels. The control voltage is fed to the IPA
module where it controls the IPA output, thus controlling the overall out-
put from the PA Module.
VHF 30/40 W Power Amplifier Modules
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