Chapter 2 Theory Of Operation; Introduction; Vhf (136-174 Mhz) 1-25W Receiver Front-End; 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 VHF 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 VHF (136–174 MHz) 1–25W Receiver Front-End
2.1

Receiver Front-End

The receiver is able to cover the VHF range from 136 to 174 MHz. 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. High-side first injection is used.
Antenna
Pin Diode
Antenna
Switch
RF Jack
Harmonic
Filter
Recovered Audio
RSSI
455kHz Filter
455kHz Filter
Varactor
RF Amp
Tuned Filter
Control Voltage
First LO
from PCIC
from FGU
Demodulator
IF
(25kHz)
(12.5kHz)
Figure 4-1 VHF Receiver Block Diagram
Varactor
Mixer
Tuned Filter
Limiter
Second LO
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
Section 4: 2-1
1. Crystal
1. IF Amp
Filter
2. Crystal
2. IF Amp
Filter
Filter Bank Selection
from Synthesizer IC
6881091C63-F