Theory Of Operation; Introduction; Vhf (136-174Mhz) Receiver; Receiver Front-End - Motorola GM Series Service Information

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 trouble shooting for the the associated Controller circuits refer to the
Controller Section of this manual.
2.0

VHF (136-174MHz) 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, high-IF, low-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 first injection is used.
Antenna
Pin Diode
Antenna
Switch
RF Jack
Harmonic
Filter
Recovered Audio
RSSI
455kHz Filter
455kHz Filter
Varactor
RF Amp
Tuned Filter
Control Voltage
from PCIC
Demodulator
IF
(25kHz)
(12.5kHz)
Figure 2-1 VHF Receiver Block Diagram

THEORY OF OPERATION

Varactor
Tuned Filter
First LO
from FGU
Limiter
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
Chapter 2
1. Crystal
Mixer 1. IF Amp
Filter
2. Crystal
2. IF Amp
Filter
Second LO
Filter Bank Selection
from Synthesizer IC