Motorola CDM1250 Service Manual

Section 9: 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 this manual.
2.0 UHF Band 1 (403–470 MHz) 25–40 W Bipolar Receiver Front-End
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, 1st IF, 2nd 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 1st IF of 44.85 MHz. Low side first injection is used.
Figure 9-1 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
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

Motorola CDM1250 Specifications

General

Display	Alphanumeric
IP Rating	IP54
Frequency Range	VHF: 136-174 MHz, UHF: 403-470 MHz
Signaling	PL, DPL, MDC1200, Quick Call II
Operating Temperature	-22°F to +140°F (-30°C to +60°C)
Power Output	VHF: 1-25W; UHF: 1-25W