Motorola pro7100 Service Manual Page 32

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2-18 Theory of Operation
Figure 2-7. UHF Receiver Block Diagram
The 2-pole 44.85 MHz crystal filters in the 1st IF section and two pairs of 455 kHz ceramic filters in
the 2nd IF section provide the required adjacent channel selectivity. The correct pair of ceramic filters
for 12.5 or 25 kHz channel spacing is selected via control line BWSELECT. The 2nd IF at 455 kHz is
mixed, amplified, and demodulated in the IF IC. The processing of the demodulated audio signal is
performed by an audio processing IC located in the controller section.
Demodulator
1. Crystal
Filter
Mixer
Varactor
Tuned Filter
RF Amp
Varactor
Tuned Filter
Pin Diode
Antenna
Switch
RF Jack
Control Voltage
from PCIC
First LO
from FGU
Recovered Audio
RSSI
Second LO
2. Crystal
Filter
455kHz Filter
(25kHz)
455kHz Filter
(25kHz)
455kHz Filter
(12.5kHz)
455kHz Filter
(12.5kHz)
Switch
Switch
Switch
Switch
Limiter
1. IF Amp
2. IF Amp
Filter Bank Selection
from Synthesizer IC
Harmonic
Filter
BWSELECT
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