Motorola Professional GP680 서비스 정보 - 페이지 13

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UHF Receiver

3.0

UHF Receiver

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

diagram of the 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 UHF Receiver Front End schematic diagram)

The RF signal received by the antenna is applied to a low-pass filter. For UHF, the filter consists of

components L101, L102, C102, C103, and C104. The filtered RF signal is passed through the

antenna switch circuit consisting of two pin diodes (CR101 and CR102) and a pi network (C106,

L104, and C107). The signal is then applied to a varactor tuned bandpass filter.

The UHF bandpass filter consists of components L301, L302, C302, C303, C304, CR301, and

CR302. The filter is electronically tuned by DACRx from IC 404 which supplies a control voltage to

the varactor diodes (CR301 and CR302) in the filter as determined by the microprocessor

depending on the carrier frequency. Wideband operation of the filter is achieved by shifting the

bandpass filter across the band.

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 UHF Receiver Block Diagram.

Varactor

Tuned Filter

AGC

Demodulator

SPI Bus

Crystal

Mixer

Filter

Synthesizer

Second

LO VCO

2-3