Section 13: 2-1
6881091C63-F
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 the manual.
2.0 UHF Band 2 (450–512/520 MHz) 25–40 W LDMOS Receiver Front-
End
2.1 Receiver Front-End
The UHF receiver, shown in Figure 13-1, 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.
Figure 13-1 UHF Receiver Block Diagram
Demodulator
1. Crystal
Filter
Mixer
Varactor
Tuned Filter
RF Amp
Varactor
Tuned Filter
Pin Diode
Antenna
Switch
RF Jack
Control Voltage
from PCIC
First LO
from FGU
Recovered Audio
RSSI
Second LO
2. Crystal
Filter
455 kHz Filter
(25 kHz)
455 kHz Filter
(25 kHz)
455 kHz Filter
(12.5 kHz)
455 kHz Filter
(12.5 kHz)
Switch
Switch
Switch
Switch
Limiter
1. IF Amp
2. IF Amp
Filter Bank Selection
from Synthesizer IC
Harmonic
Filter
BWSELECT
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