Motorola GP328 Plus, Руководство по обслуживанию

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5A-6 Receiver
5.0 Receiver
5.1 Receiver Front-End
(Refer to VHF Receiver Front End Schematic Diagram on page 5A-22, VHF Receiver Back End
Schematic Diagram on page 5A-23, and VHF Transmitter Schematic Diagram on page 5A-26)
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.
The output of the bandpass filter is coupled to the RF amplifier transistor Q3302 via C3306. After
being amplified by the RF amplifier, the RF signal is further filtered by a second varactor tuned
bandpass filter, consisting of L3305, L3306, C3311 to C3314 and D3302.
Both the pre and post-RF amplifier varactor tuned filters have similar responses. The 3 dB bandwidth
of the filter is about 12 MHz. This enables the filters to be electronically controlled by using a single
control voltage which is DACRx .
Figure 5-2: 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