Chapter 2 Theory Of Operation; Introduction; Major Assemblies; Receiver - Motorola EP450 Basic Service Manual

Chapter 2 Theory Of Operation

2.1

Introduction

This chapter provides a basic theory of operation for the radio components.
2.2

Major Assemblies

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
2.2.1

Receiver

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
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.
More user manuals on
• 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).
(Figure 2-1).
Antenna
3
Battery
2
Spkr

Figure 2-1. Major Assemblies Block Diagram

(Figure 2-2).
ManualsBase.com
M140
Keypad
Transceiver
Board
Board
28
J301 J460 J2
J491
J470
2
Mic
Display
J1