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

Transmitter Power Amplifier (PA) 40W
3.6

Antenna Switch

The antenna switch utilizes the existing dc feed (A+) to the last stage device (Q5441). Basic
operation is to have both PIN diodes (D5471 and D5472) turned on during key-up by forward
biasing them. This is achieved by pulling down the voltage at the cathode end of D5472 to around
11.8V (0.7V drop across each diode). The current through the diodes needs to be set around 80mA
to fully open the transmit path through resistor R5496. Q5472 is a current source controlled by
Q5471 and is eventually connected to pin ANO of PCIC. VR5471 ensures the voltage at resistor
R4511 never exceeds 5.6V.
3.7

Harmonic Filter

Inductors L5491, L5492, and L4493 along with capacitors C5448, C5493, C5494, C5496, and
C5498 form a low-pass filter to attenuate harmonic energy from the transmitter. Resistor R5491
drains any electrostatic charges 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
to improve spurious response rejection.
3.8

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 PCIC has internal digital-to-analog converters (DACs) which provide a reference voltage to the
control loop. The reference voltage level is programmable through the SPI line of the PCIC and is
proportional to the desired power setting of the transmitter. Factory programming at several points
across the frequency range of the transmitter is used to offset frequency response variations of the
transmitter's power detector circuits.
The PCIC provides a dc output voltage at pin 4 (INT) and applied as CNTLVLTG to the power-adjust
input pin of the first transmitter stage U5401. This adjusts the transmitter power output to the
intended value. Variations in forward or reflected 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.
Capacitor 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 29)
proportional to temperature. If the dc voltage produced exceeds the set threshold in the PCIC, the
transmitter output power is reduced to decrease the transmitter temperature.
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