Service manual

Chapter 5 146-174 MHz VHF Theory Of Operation

5.1 Introduction

This chapter provides a detailed theory of operation for the radio components. Schematic diagrams

for the circuits described in the following paragraphs are located in Chapter 7 of this manual.

5.2 VHF Receiver

The VHF receiver covers the range of 146-174 MHz and provides switchable IF bandwidth for use

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

shown in Figure 5-1.

•Front End

• Back End

Figure 5-1. VHF Receiver Block Diagram

5.2.1 Receiver Front-End

Incoming RF signals from the antenna are first routed through the harmonic filter and antenna switch,

part of the transmitter circuitry, before being applied to the receiver front end. The receiver front end

consists of a preselector filter, RF amplifier, an interstage filter, and a double-balanced first mixer.

The preselector filter is a fixed-tuned 4-pole design using discrete elements (L1-L4 and C1-C9) in a

series/shunt resonator configuration. It has a 3 dB bandwidth of 44 MHz, an insertion loss of 2 dB and

image attenuation of 40 dB at 235 MHz, with increasing attenuation at higher frequencies. Diode CR1

protects the RF amplifier by limiting excessive RF levels.

The output of the filter is matched to the base of RF amplifier Q21, which provides 18 dB of gain and

a noise figure of 2 dB. Operating voltage is obtained from the 5R source, which is turned off during

transmit to reduce dissipation in Q21. Current mirror Q22 maintains the operating current of Q21

Demodulator

Crystal

Filter

1st Mixer

Amp

Preselector

Filter

Interstage

Filter

Recovered Audio

RSSI

RX from

Antenna Switch

Inj Filter

First LO

from Synthesizer

Ceramic

Resonator

Cer Fltr

Switching

BW_SEL