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UHF (403-470MHz) Transmitter Power Amplifier (PA) 40W
5.8
Power Control
The transmitter uses the Power Control IC (PCIC, U4501) to control the power output of the radio. A
portion of the forward RF power from the transmitter is sampled by the bi-directional coupler and
rectified, to provide a DC voltage to the RFIN port of the PCIC (pin 1) which is proportional to the
sampled RF power.
The PCIC has internal digital to analog converters (DACs) which provide the reference voltage of the
control loop. The reference voltage level is programmable through the SPI line of the PCIC. This
reference voltage is proportional to the desired power setting of the transmitter, and is factory
programmed at several points across the frequency range of the transmitter to offset frequency
response variations of the transmitter s power detector circuitry.
The PCIC provides a DC output voltage at pin 4 (INT) which is applied as CNTLVLTG to the power-
adjust input pin of the first transmitter stage U4401. This adjusts the transmitter power output to the
intended value. Variations in forward transmitter power cause the DC voltage at pin 1 to change, and
the PCIC adjusts the control voltage above or below its nominal value to raise or lower output power.
Capacitors C4502-4, in conjunction with resistors and integrators within the PCIC, control the
transmitter power-rise (key-up) and power-decay (de-key) characteristic to minimize splatter into
adjacent channels.
U4502 is a temperature-sensing device, which monitors the circuit board temperature in the vicinity
of the transmitter driver and final devices, and provides a dc voltage to the PCIC (TEMP, pin 29)
proportional to temperature. If the DC voltage produced exceeds the set threshold in the PCIC, the
transmitter output power will be reduced so as to reduce the transmitter temperature.
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