Temperature Cut Back Circuit - Motorola GP328 Detailed Service Manual

Transmitter
3.1.2 Antenna Switch
The antenna switch circuit consists of two pin diodes (D100 and D101), a RF network (C147 and
L103), and a DC feed network (L104, C144 and current limiting resistor R101). In the transmit mode,
PCIC (U102) pin 32 goes high supplying current via the feed network to bias the diodes "on". The
shunt diode (D101) shorts out the receiver port and L103 is connected from the RF path to ground.
L103 and the input capacitance of the lowpass filter form a parallel resonant circuit, effectively
disconnecting the receiver port from the antenna while not loading the transmit path. In the receive
mode, pin 32 goes low and the.the diodes are off. D100 looks like a high impedance disconnecting
the transmitter from the antenna while L103 and C147 form a series resonant circuit to connect the
receiver to the antenna.
3.1.3 Harmonic Filter
The harmonic filter consists of components C103, C106, C107,C110, C111, C114, C115 and 3
inductors which are a part of the SH100 assembly. The harmonic filter for lowband is pole zero design
which give greater attenuation lower in frequency where the harmonic energy of the transmitter is the
greatest and less attenuation high in frequency where there is less harmonic energy. The harmonic
filter insertion loss is typically less than 0.8 dB
3.1.4 Antenna Matching Network
The antenna matching network (T100) matches the antenna's impedance with the harmonic filter to
optimize the performance of the transmitter and receiver.
3.1.5 Power Control Integrated Circuit (PCIC)
The transmitter uses the PCIC (U102) to regulate the power output of the radio. To accomplish this,
the voltage across R102 is sensed. This voltage drop is directly proportional to the current drawn in
the final stage of the transmitter. This voltage is compared to a programmable reference inside the
PCIC and the voltage on PCIC pin 4 adjusted. Pin 4 connects to the PA driver IC (U101) pin 1 via
resistor R100 and varies RF output power of the driver. This controls the current drain of the final
stage and sets the output power.

3.1.6 Temperature Cut Back Circuit

Temperature sensor VR101 and associated components are part of a temperature cut back circuit.
This circuit senses the printed circuit board temperature around the transmitter circuits and outputs a
DC voltage to the PCIC. If the DC voltage produced exceeds the set threshold of the PCIC, the
transmitter output power decreases to reduce the transmitter temperature.
6E-5