Motorola EP450 service manual Theory Of Operation, Introduction, Major Assemblies

Chapter 2 Theory Of Operation

2.1Introduction

This chapter provides a basic theory of operation for the radio components.

2.2Major Assemblies

•Transceiver Board – contains all transmit, receive, and audio circuitry.

•Display (Display models only) – 8 characters (14 segments star burst) and 10 icons with back- lighting, liquid-crystal display (LCD).

Keypad Board (Display models only) –

Display model: a 2-button menu keypad with 2-way navigation button, and a 3 x 4 alphanumeric keypad.

Transceiver Board (Figure 2-1).

Antenna

Battery

Spkr

3

2

M140

Transceiver

Board

J301 J460

J491

J470

28

Keypad

Board

J2 J1

Display

2

Mic

Figure 2-1. Major Assemblies Block Diagram

2.2.1Receiver

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, interstage filter, and a double-balanced first mixer (Figure 2-2).

The mixer output is applied to a diplexer network which matches the 44.85 MHz IF signal to the crystal filter, and terminates the mixer into 50 ohms at all other frequencies.

The receiver back-end is a dual conversion design. High IF selectivity is provided by a 4-pole fundamental mode 44.85 MHz crystal filter. The output is matched to an IF amplifier stage. The output of the IF amplifier is applied to the input of the receiver IFIC.