Chapter 2 Theory Of Operation; Introduction; Major Assemblies; Receiver - Motorola Commercial Series 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

• Transceiver Board – contains all transmit, receive, and audio circuitry.
• Display (Limited and Full Keypad models only) – 8 characters (14 segments star burst) and 10
icons with backlighting, liquid-crystal display (LCD).
Keypad Board (Limited and Full Keypad models only) –
Limited Keypad: a 2-button menu keypad with 2-way navigation button,
Full Keypad: a 2-button menu keypad with 2-way navigation button, and a 3 x 4 alphanumeric
keypad.Transceiver Board
Antenna
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 (Figure
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.
(Figure 2-1).
M140
Transceiver
Board
3
Battery
J301
2
Spkr
J491
J470
2
Mic

Figure 2-1. Major Assemblies Block Diagram

2-2).
Keypad
Board
28
J460 J2
Display
J1