Antenna Switch; Harmonic Filter; Power Control - Motorola GM328 Service Manual

UHF (450-520 MHz) Transmitter Power Amplifier (PA) 40W
3.5

Antenna Switch

The antenna switch consists of two PIN diodes, D5471 and D5472. In the receive mode, both
diodes are off. Signals applied at the antenna jack J5401 are routed, via the harmonic filter, through
network L5472, C5474 and C5475, to the receiver input. In the transmit mode, K9V1 turns on
Q5471 which enables current sink Q5472, set to 96 mA by R5512 and VR5471. This completes a
DC path from PASUPVLTG, through L5437, D5471, L5472, D5472, L5473, R5496 and the current
sink to ground. Both diodes are forward biased into conduction. The transmitter RF from the
directional coupler is routed via D5471 to the harmonic filter and antenna jack. D5472 also
conducts, shunting RF power and preventing it from reaching the receiver port (RXIN). L5472 is
selected to appear as a broadband lambda/4 wave transmission line, making the short circuit
presented by D5472 appear as an open circuit at the junction of D5472 and the receiver path.
3.6

Harmonic Filter

Components L5491-L5493 and L5472, C5448, C5494, C5496 and C5498 form a Butterworth low-
pass filter to attenuate harmonic energy of the transmitter to specifications level. R5491 is used to
drain electrostatic charge that might otherwise build up on the antenna. The harmonic filter also
prevents high level RF signals above the receiver passband from reaching the receiver circuits,
improving spurious response rejection.
3.7

Power Control

The transmitter uses the power control IC (PCIC, U5501) 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) that is proportional to the
sampled RF power.
The ASFIC (U0221) has internal digital to analog converters (DACs) which provide a reference
voltage of the control loop to the PCIC via R5483. 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 circuit.
The PCIC provides a DC output voltage at pin 4 (INT) which sets the drain current of the first
(U5401). 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 C5502-4, 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.
U5502 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 30)
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 decrease the transmitter temperature.
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