Antenna Switch; Harmonic Filter; Antenna Matching Network; Power Control Integrated Circuit (Pcic) - Motorola Professional Radio Service Manual

2-6
2.5.2

Antenna Switch

The antenna switch circuit consists of two pin diodes (CR101 and CR102), a pi network (C107, L104
and C106), and two current limiting resistors (R101 and R170). In the transmit mode, B+ at PCIC
(U102 pin 23) goes low turning on Q111, which applies a B+ bias to the antenna switch circuit to bias
the diodes "on". The shunt diode (CR102) shorts out the receiver port and the pi network. This
operates as a quarter wave transmission line to transform 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,
creating a low attenuation path between the antenna and receiver ports.
2.5.3

Harmonic Filter

The harmonic filter consists of components C104, L102, C103, L101 and C102. The harmonic filter
for UHF is a modified Zolotarev design 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.
2.5.4

Antenna Matching Network

The antenna matching network, which is made up of L116, matches the antenna's impedance with the
harmonic filter to optimize the performance of the transmitter and receiver.
2.5.5

Power Control Integrated Circuit (PCIC)

The transmitter uses the PCIC (U102) to regulate the power output of the radio. To accomplish this,
the current to the final stage of the power module, supplied through R101, provides a voltage
proportional to the current drain. This voltage is then fed back to the automatic level control (ALC)
within the PCIC to regulate the output power of the transmitter.
The PCIC contains internal digital to analog converters (DACs) that provide a programmable control
loop reference voltage.
The PCIC internal resistors, integrators, and external capacitors (C133, C134 and C135) control the
transmitter rise and fall times to reduce the power splatter into adjacent channels.
2.5.6

Temperature Cut Back Circuit

Diode CR105 and associated components are part of a temperature cutback 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.
2.6

UHF Receiver

The UHF receiver consists of a front end, back end, and automatic gain control circuits. A block
diagram of the receiver is shown in Figure 2-6. Detailed descriptions of these stages are contained in
the paragraphs that follow.