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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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