Theory Of Operation; Introduction; Receiver Front-End - Motorola CDM Series Detailed Service Manual

Chapter 2

THEORY OF OPERATION

1.0

Introduction

This chapter provides a detailed theory of operation for the UHF circuits in the radio. For details of
the theory of operation and troubleshooting for the associated controller circuits refer to the
controller section of this manual.
2.0 UHF Band 2 (450–512/520 MHz) 25–40 W Bipolar Receiver Front-
End
2.1

Receiver Front-End

The UHF receiver, shown in Figure 10-1, is able to cover the UHF range from 450 to 512 MHz (for
CDM models) and from 450 to 520 MHz (for PRO models). It consists of four major blocks: front-end
bandpass filters and pre-amplifier, first mixer, high-IF, low-IF and receiver back-end. Two varactor-
tuned bandpass filters perform antenna signal pre-selection. A passive double-balanced mixer
converts the signal to the first IF of 44.85 MHz. Low-side first injection is used.
Pin Diode
Antenna
Switch
RF Jack
Harmonic
Filter
Recovered Audio
RSSI
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
Varactor
RF Amp
Tuned Filter
Control Voltage
First LO
from PCIC
from FGU
Demodulator
Limiter
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
BWSELECT
Figure 10-1 UHF Receiver Block Diagram
Varactor
Mixer
Tuned Filter
Second LO
Section 10: 2-1
1. Crystal
1. IF Amp
Filter
2. Crystal
2. IF Amp
Filter
Filter Bank Selection
from Synthesizer IC
6881091C63-F