Motorola GM380 Service Information - Page 11

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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

Varactor

RF Amp

Tuned Filter

Control Voltage

from PCIC

Demodulator

(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