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Si4770/77-A20
4.4. FM Receiver Front-End
The Si4770/77-A20 provides a very flexible front-end
interface to accommodate a wide range of applications
from cost-sensitive to high-performance.
An advanced AGC on the Si4770/77-A20 is
implemented with the use of internal RF peak and IF
peak detectors with programmable thresholds (trip
points). The AGC adjusts the resistor values
automatically. Attack and release rates for the AGC are
programmable, providing flexible fast attack and slow
release AGC performance.
FMXIP
FMXIN
RF Pkd
For cost-effective performance and superior FM
sensitivity, the antenna output can be received on the
FMI pin (Figure 9). The FM band can be received on the
FMI pin via an input coupling network. This input
coupling network isolates the FM band for best
performance. An internal LNA provides gain for the
signal. The LNA output is routed externally to the FM
50 ?
RFREG
FMO
FMI
Reg
R L
LNA
RF Pkd
Si477x
mixer input pins. The LNA gain is regulated with an
internal voltage regulator supply via an internal resistor
bank, R L . The AGC circuit automatically controls the
LNA gain, resistor banks FMAGC1, FMAGC2, and R L to
optimize sensitivity and strong signal handling.
FMXIP
Figure 10. Conceptual Illustration of the
Lowest-Cost Configuration
4.4.1. FMI LNA for FM Loop-Through Usage
In dual receiver applications, two receivers (Figure 11)
can be attached to a single antenna. The dual receiver
solution allows for independent radio station listening in
different rooms.
The FMI LNA input impedance is software-configurable
and provides two options: 50 ? and 100 ? . Configuring
the input impedance for 100 ? facilitates a Si4770/77-
A20 receiver 1 and the Si4770/77-A20 receiver 2 to be
interfaced to the antenna output in parallel, providing a
matched 50 ? input impedance. AGC is coordinated
between both receivers whereby the resistor banks,
FMXIN
RFREG
R L
RF Pkd
Reg
FMAGC1, FMAGC2, and R L , from one receiver are
used to optimize sensitivity and strong signal handling.
50 ?
FMO
FMI
LNA
RF Pkd
Si477x
Figure 9. Conceptual Illustration of the Use of
the FMI LNA for Cost-Optimized and Superior
FM Sensitivity Performance
Cost can be further reduced by eliminating the 1:1 balun
and directly interfacing the signal to the FM mixer by
programming the mixer for single-ended input mode
(Figure 10). The trade-off is a drop in linearity of 6 dBμV
in IP3.
26
Rev. 0.9
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