Motorola Professional GP640 Сервісна інформація - Сторінка 13

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Motorola Professional GP640 Сервісна інформація

VHF Receiver

3.0

VHF Receiver

The VHF receiver consists of a front end, back end, and automatic gain control circuits. A block

diagram of the VHF receiver is shown in Figure 2-2. Detailed descriptions of these features are

contained in the paragraphs that follow.

Antenna

Pin Diode

Antenna

Switch

RF Jack

3.1

Receiver Front-End

(Refer to Figure 2-2 and the VHF Receiver Front End schematic diagram)

The RF signal is received by the antenna and applied to a low-pass filter consisting of L3531, L3532,

C3532 to C3563. The filtered RF signal is passed through the antenna switch. The antenna switch

circuit consists of two pin diodes (D3521 and D3551) and a pi network (C3531, L3551, and

C3550).The RF signal is then applied to a varactor tuned bandpass filter which consists of L3301,

L3303, C3301 to C3304, and D3301. The filter is tuned by applying a control voltage to the varactor

diode (D3301) in the filter.

The bandpass filter is electronically tuned by the DACRx from IC 404 which is controlled by the

microprocessor. Depending on the carrier frequency, the DACRx supplies the tuned voltage to the

varactor diodes in the filter. Wideband operation of the filter is achieved by shifting the bandpass

filter across the band.

The output of the bandpass filter is coupled to the RF amplifier transistor Q3302 via C3306. After

being amplified by the RF amplifier, the RF signal is further filtered by a second varactor tuned

bandpass filter, consisting of L3305, L3306, C3311 to C3314 and D3302.

Varactor

RF Amp

Tuned Filter

First LO

from FGU

Control Voltage

from ASFIC

Recovered Audio

Squelch

RSSI

16.8 MHz

Reference Clock

Figure 2-2 VHF Receiver Block Diagram.

Varactor

Tuned Filter

AGC

Demodulator

SPI Bus

Crystal

Mixer

Filter

Synthesizer

Second

LO VCO

2-3