Motorola GM338

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2-1

Section 2

THEORY OF OPERATION

1.0 Introduction

This Chapter provides a detailed theory of operation for the Low Band 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 Low Band Receiver

2.1 Receiver Front-End

The low band receiver, shown in Figure 2-1, is bandsplit into three ranges depending on radio

model, covering frequencies from 29.7 to 36.0 MHz, 36.0 to 42.0 MHz, or 42.0 to 50.0 MHz. The

circuitry of the three models is identical except for component value differences. The receiver

consists of five major blocks: front-end bandpass filters and pre-amplifier, first mixer, high-IF and

blanker switches, low-IF and receiver back-end, and “Extender” (noise blanker). Two fixed-tuned

bandpass filters perform antenna signal pre-selection. A cross over quad diode mixer converts the

signal to the high-IF of 10.7 MHz. High-side first injection is used.

Figure 2-1 Low Band Receiver Block Diagram

Demodulator

Delay

Filter

Mixer

Fixed

Tuned Filter

RF Amp

Tuned Filter

Pin Diode

Antenna

Switch

RF Jack

Antenna

First LO

RXINJ

Recovered Audio

RSSI

Second LO

Crystal

Filter

455kHz Filter

(12.5kHz)

455kHz Filter

(12.5kHz)

Switch

Limiter

IF Amp

Filter Bank Selection

from Synthesizer IC

Blanker

Noise

Blanking Pulses

Gate

Buffer

Enable

1st

Fixed

(25/20kHz)

BWSELECT

(U1201 Pin 48)

4-pole

(25/20kHz)

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