Motorola Professional GP680 Service-informatie - Pagina 11

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Motorola Professional GP680 Service-informatie

Receiver

3.0

Receiver

Antenna

Pin Diode

Antenna

Switch

RF Jack

3.1

Receiver Front-End

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

The RF signal is received by the antenna and applied to a low-pass filter. For 300R1, the filter

consists of L101, L102, C102, C103, C104. The filtered RF signal is passed through the antenna

switch. The antenna switch circuit consists 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 VHF

bandpass filter comprises of L301, L302, C302, C303, C304, CR301 and CR302. The bandpass

filter is tuned by applying a control voltage to the varactor diodes (CR301 and CR302) in the filter.

The bandpass filter is electronically tuned by the DACRx from U404 which is controlled by the

microprocessor. Depending on the carrier frequency, the DACRx will supply 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 Q301 via C307. After being

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

filter, consisting of L306, L307, C313, C317, CR304 and CR305.

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 Receiver Block Diagram

Varactor

Tuned Filter

AGC

Demodulator

U301

SPI Bus

Crystal

Mixer

Filter

Synthesizer

Second

LO VCO

2-3

IF Amp