SI4703 데이터 시트보기 (PDF) - Silicon Laboratories
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SI4703 Datasheet PDF : 69 Pages
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AN383
To resonate the cable antenna within the FM band, the antenna’s capacitance needs to be reduced. As described
in Section “4.2. Cable Antenna Schematic”, this reduction can be achieved by placing the ferrite beads in series
with each of the antenna’s conductors. The capacitance should be further controlled by limiting the trace length
from the cable ground shield and the RF input pin (FMI input) on the Si47xx FM tuner. Each of the components in
the schematic above is explained in detail below:
L1 (27 0nH) is the tuning inductor. This is the typical value used to resonate the cable antenna in the center of the
FM band.
F1 (2.5 k at 100 MHz) is a shunt ferrite to ground at the cable antenna side. A substantial amount of ground
return current may flow through the cable antenna shield/ground because there are multiple conductors inside the
cable along with power supply conductors. The ferrite will divert the ground return current of the cable antenna to
go through the shunt ferrite rather than going through the tuning inductor and/or Si47xx chip.
C1 (100 pF) is a dc blocking cap placed between the FMI pin and the cable antenna ground. The capacitor is used
to isolate the cable return currents from the FMI pin.
F2 (2.5 k at 100 MHz) is a series ferrite placed on the signal conductor in the cable antenna. Note that series
ferrites should be placed on each signal conductor in the cable. The ferrite is used to isolate the signal conductors
from the shield/ground of the cable antenna and reduce parasitic capacitance seen by the shield/ground. The
choice of the ferrite is dependent upon the type of signal on each individual conductor. If the conductor is used to
carry power, then a ferrite with a large dc current carrying capability should be used. If the conductor is used to
carry high frequency digital signals, make sure that the ferrite does not block the high frequency component of
these signals. Likewise if the conductor is used to carry high frequency analog signals, make sure that the ferrite
does not filter the high frequency.
4.3. Cable Antenna Bill of Materials
The required bill of materials is shown below:
Table 13. Bill of Materials
Designator
L1
C1
F1
F2
Description
IND, 0603, SM, 270 nH, MURATA,
LQW18ANR27J00D
DC blocking capacitor, SM, 0402, X7R, 100 pF
Shunt Ferrite bead, FERRITEBEAD, SM, 0603,
470 Ω, 1 A, Murata, BLM18PG471SN1J
Series Ferrite bead, various types.
Recommended ferrite for power lines:
FERRITEBEAD, SM, 0603, 470 Ω, 1 A, Murata,
BLM18PG471SN1J
Recommended ferrite for signals: FERRITEBEAD,
SM, 0603, 2.5 kΩ, 50 mA Murata,
BLM18BD252DN1D
Notes
Rated dc current > max expected
ground return current
For power signals, make sure the
rated dc current > max expected
ground return current.
For all other signals, make sure
ferrite does not block/filter the high
frequency component of the signals.
Rev. 0.8
23
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