SI4770-77-A20 Datasheet, PDF (27/54 Pages) Silicon Laboratories – HIGH-PERFORMANCE CONSUMER ELECTRONICS

Si4770/77-A20
4.5. AM Receiver Front-End
The Si4770/77-A20 contains an integrated LNA,
providing an AM receive chain from antenna to audio
out. There are few external components and no manual
alignment required. The AM signal is received on the
AMI pin. An advanced AGC on the Si4770/77-A20 is
implemented with the use of internal RF peak and IF
FMXIP
RF Pkd
peak detectors with programmable thresholds (trip
points). Attack and release rates for the AGC are
FMXIN
RFREG
Reg
RL
programmable providing flexible fast attack and slow
release AGC performance.
The Si4770/77-A20 provides highly-accurate digital AM
tuning without factory adjustments. To offer maximum
50 
FMO
LNA
flexibility, the receiver supports a wide range of ferrite
100 
FMI
RF Pkd
loop sticks from 180~688 µH. An air loop antenna is
supported by using a transformer to increase the
Si477x
effective inductance of the air loop. Using a 1:5 turn
ratio inductor, the inductance is increased by 25 times
and easily supports all typical AM air loop antennas
which generally vary between 10 and 20 µH.
FMXIP
FMXIN
RFREG
FMO
RF Pkd
Reg
R_L
LNA
100  ^FMI
RF Pkd
Si477x
Figure 11. Conceptual Illustration of Si4770/77-
A20 Receivers Interfaced to a Single Antenna
Using the FMI LNA in Loop-Through Mode
Rev. 0.9
27