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1.0

Introduction

This Chapter provides a detailed theory of operation for the VHF circuits in the radio. Schematic

diagrams and board layout diagrams are included in Chapter 4 in this Section of the manual.

2.0

VHF (136-162MHz) Receiver

The VHF receiver covers the range of 136-162 MHz and provides switchable IF bandwidth for use

with 20/25/30 kHz or 12.5 kHz channel spacing systems. The receiver is divided into two major

blocks, as shown in Figure 2-1.

■

Front End

■

Back End

RX from

Antenna Switch

Preselector

Filter

Amp

First LO

from Synthesizer

Recovered Audio

RSSI

Figure 2-1 VHF Receiver Block Diagram

THEORY OF OPERATION

Interstage

Crystal

Filter

1st Mixer

Inj Filter

Ceramic

Resonator

Demodulator

12.5kHzFilter

Chapter 2

Amp

Filter

BW_SEL

Cer Fltr

Switching