Theory Of Operation; Introduction; Vhf (136-174 Mhz) Receiver; Receiver Front-End - Motorola GM328 Service Manual

Section 2

THEORY OF OPERATION

1.0

Introduction

This Chapter provides a detailed theory of operation for the VHF 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

VHF (136-174 MHz) Receiver

2.1

Receiver Front-End

The receiver is able to cover the VHF range from 136 to 174 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 first IF of 44.85 MHz. High-side injection is used..
Pin Diode
Antenna
Switch
RF Jack
Recovered Audio
RSSI
Varactor
Tuned Filter
Control Voltage
from PCIC
Demodulator
IF
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
Figure 2-1 VHF 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