Receiver
1-3
U3503 and its associated circuitry acts as a temperature cut back circuitry. This circuitry provides
the necessary voltage to the PCIC to cut the transmitter power when the radio temperature gets too
high.
3.0
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 .
3.1
Receiver Front-End
The RF signal is received by the antenna and applied to a low-pass filter. For VHF, the filter consists
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 signal is then applied to a varactor tuned bandpass filter. The VHF bandpass
filter comprises of L3301, L3303, C3301 to C3304 and D3301. The bandpass 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 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.
Figure 1-2
VHF Receiver Block Diagram
Demodulator
Synthesizer
Crystal
Filter
Mixer
Varactor
Tuned Filter
RF Amp
Varactor
Tuned Filter
Pin Diode
Antenna
Switch
RF Jack
Antenna
AGC
Control Voltage
from ASFIC
First LO
from FGU
Recovered Audio
Squelch
RSSI
IF
IC
SPI Bus
16.8 MHz
Reference Clock
Second
LO VCO
IF Amp
U3220
Summary of Contents for GP-688
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