Uhf2 Receiver (Models With Pcb 8486686Z02); Receiver Front-End - Motorola GP Series Service Manual

2-6
4.0 UHF Receiver (Models with PCB 8486686Z02)
The UHF receiver consists of a front and back end. A block diagram of the receiver is shown in
Figure 2-3. Detailed descriptions of these features are contained in the paragraphs that follow.
Antenna
Pin Diode
Antenna
Switch
RF Jack
4.1

Receiver Front-End

( Refer to Figure 2-3 and the UHF Receiver Front End schematic diagram for PCB 8486686Z02)
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.
Varactor
RF Amp
Tuned Filter
Control Voltage
from ASFIC
Demodulator
Recovered Audio
RSSI
BW SEL
Figure 2-3 UHF Receiver Block Diagram.
Varactor
Tuned Filter
First LO
from VCO
Limiter
455kHz
Filter
455kHz
Filter
THEORY OF OPERATION
Crystal
Mixer
Filter
U301
455kHz
Filter
Second LO
IF Amp