Motorola LB2 (36.0 - 42.0MHz) Сервісна інформація - Сторінка 8

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Motorola LB2 (36.0 - 42.0MHz) Сервісна інформація

2-2

2.0

Low Band Receiver

2.1

Receiver Front-End

The low band receiver 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.

Enable

Antenna

Pin Diode

Antenna

Switch

RF Jack

Recovered Audio

RSSI

Blanking Pulses

Noise

Blanker

Fixed

Tuned Filter

Demodulator

455kHz Filter

(25/20kHz)

455kHz Filter

(12.5kHz)

Figure 2-1 Low Band Receiver Block Diagram

Delay

IF Amp

Filter

Fixed

RF Amp

Tuned Filter

First LO

RXINJ

Limiter

455kHz Filter

(25/20kHz)

455kHz Filter

(12.5kHz)

THEORY OF OPERATION

Buffer

Gate

1st

Mixer

4-pole

Crystal

IF Amp

Second LO

BWSELECT

Filter Bank Selection

from Synthesizer IC

(U1201 Pin 48)

Filter