Motorola GM328 Service Manual page 417

Section 2
THEORY OF OPERATION
1.0 Introduction
This Chapter provides a detailed theory of operation for the UHF circuits in the radio. For details of
the theory of operation and troubleshooting for the the associated Controller circuits refer to the
Controller Section of this manual.
2.0 UHF (403-470 MHz) Receiver
2.1 Receiver Front-End
The receiver is able to cover the UHF range from 403 to 470 MHz. It consists of four major blocks:
front-end bandpass filters and pre-amplifier, first mixer, 1st IF, 2nd IF and receiver back-end . Two
varactor tuned bandpass filters perform antenna signal pre-selection. A cross over quad diode mixer
converts the signal to the 1st IF of 44.85 MHz. Low side first injection is used.
Pin Diode
Antenna
Switch
RF Jack
Harmonic
Filter
Recovered Audio
RSSI
455kHz Filter
455kHz Filter
Varactor
Tuned Filter
Control Voltage
from PCIC
Demodulator
(25kHz)
(12.5kHz)
BWSELECT
Figure 2-1 UHF Receiver Block Diagram
Varactor
RF Amp
Tuned Filter
First LO
from FGU
Limiter
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
1. Crystal
Mixer 1. IF Amp
Filter
2. Crystal
2. IF Amp
Second LO
Filter Bank Selection
from Synthesizer IC
2-1
Filter