Motorola GM328 - Theory of Operation; Introduction; UHF (403-470 Mhz) Receiver; Receiver Front-End

2-1
Section 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 the associated Controller circuits refer to the
Controller Section of this manual.
2.0 UHF (403-470 MHz) Receiver
2.1 Receiver Front-End
The receiver is able to cover the UHF range from 403 to 470 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 cross over quad diode
mixer converts the signal to the first IF of 44.85 MHz. Low-side first injection is used.
Figure 2-1 UHF Receiver Block Diagram
Demodulator
1. Crystal
Filter
Mixer
Varactor
Tuned Filter
RF Amp
Varactor
Tuned Filter
Pin Diode
Antenna
Switch
RF Jack
Antenna
Control Voltage
from PCIC
First LO
from FGU
Recovered Audio
RSSI
Second LO
2. Crystal
Filter
455kHz Filter
(25kHz)
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
455kHz Filter
(12.5kHz)
Switch Switch
Switch
Switch
Limiter
1. IF Amp
2. IF Amp
Filter Bank Selection
from Synthesizer IC
Harmonic
Filter
BWSELECT