Chapter 2 Theory Of Operation; Introduction; Uhf (403-470Mhz) Receiver; Receiver Front-End - Motorola 6864115B62-C Detailed Service Manual

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

UHF (403-470MHz) 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, 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. Low-side first injection is used.
Antenna
Pin Diode
Antenna
Switch
RF Jack
Harmonic
Filter
Recovered Audio
RSSI
Varactor
Tuned Filter
Control Voltage
from PCIC
Demodulator
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
BWSELECT
Figure 2-1 UHF Receiver Block Diagram
THEORY OF OPERATION
Varactor
RF Amp
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
Second LO
Filter Bank Selection
from Synthesizer IC
Filter