Receiver; Receiver Front-End - Motorola GP328 Plus Service Manual

5B-6
5.0

Receiver

Antenna
Pin Diode
Antenna
Switch
RF Jack
5.1

Receiver Front-End

(Refer to UHF Receiver Front End Schematic Diagram on page 5B-21 and UHF Transmitter
Schematic Diagram on page 5B-25)
The RF signal is received by the antenna and applied to a low-pass filter. For UHF, 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 UHF 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 IC404 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.
Both the pre and post-RF amplifier varactor tuned filters have similar responses. The 3 dB bandwidth
of the filter is about 50 MHz. This enables the filters to be electronically controlled by using a single
control voltage which is DACRx .
Varactor
RF Amp
Tuned Filter
First LO
from FGU
Control Voltage
from ASFIC
Recovered Audio
Squelch
RSSI
16.8 MHz
Reference Clock
Figure 5-2: UHF Receiver Block Diagram
Varactor
Mixer
Tuned Filter
AGC
Demodulator
U301
IC
IF
SPI Bus
Crystal
IF Amp
Filter
Synthesizer
Second
LO VCO
Receiver