Theory Of Operation; Introduction; Low Band Receiver; Receiver Front-End - Motorola GM328 Service Manual

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THEORY OF OPERATION

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.

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.

Blanking Pulses

Tuned Filter

Recovered Audio

455kHz Filter

455kHz Filter

Figure 2-1 Low Band Receiver Block Diagram

Tuned Filter

455kHz Filter

455kHz Filter

Filter Bank Selection

from Synthesizer IC

(U1201 Pin 48)

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