Motorola GP series, Detailed Service Manual

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1-4 THEORY OF OPERATION
3.1.2 Antenna Switch
The antenna switch circuit consists of two PIN diodes (CR101 and CR102), a pi network (C107, L104
and C106). In the transmit mode, B+ at PCIC (U102) pin 23 will go low and turn on Q111 where a B+
bias is applied to the antenna switch circuit to bias the diodes "on". The shunt diode (CR102) shorts
out the receiver port, and the pi network, which operates as a quarter wave transmission line,
transforms the low impedance of the shunt diode to a high impedance at the input of the harmonic
filter. In the receive mode, the diodes are both off, and hence, there exists a low attenuation path
between the antenna and receiver ports.
3.1.3 Harmonic Filter
The harmonic filter consists of L103, C131, C139, C104, L102, C103, L101 and C102. The design of
the harmonic filter for UHF is that of a modified Zolotarev design. It has been optimized for efficiency
of the power module. This type of filter has the advantage that it can give a greater attenuation in the
stop-band for a given ripple level. The harmonic filter insertion loss is typically less than 1.2dB.
3.1.4 Antenna Matching Network
A matching network which is made up of L116 is used to match the antenna's impedance to the
harmonic filter. This will optimize the performance of the transmitter and receiver into an antenna.
3.1.5 Power Control Integrated Circuit (PCIC)
The transmitter uses the Power Control IC (PCIC), U102 to regulate the power output of the radio.
The current to the final stage of the power module is supplied through R101, which provides a voltage
proportional to the current drain. This voltage is then fedback to the Automatic Level Control (ALC)
within the PCIC to regulate the output power of the transmitter.
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.
There are resistors and integrators within the PCIC, and external capacitors (C133, C134 and C135)
in controlling the transmitter rising and falling time. These are necessary in reducing the power
splatter into adjacent channels.