AN851 Datasheet, PDF (9/25 Pages) Silicon Laboratories – Si468X SCHEMATICS AND LAYOUT GUIDE

AN851
4.1.3. Headphone Antenna Layout
To minimize inductive and capacitive coupling, inductors C13, L1 and L2 should be placed together close to the
Si468x and as far from noise sources such as clocks and digital circuits as possible.
To minimize shunt capacitance on antenna trace, place ferrite beads F1 and F2 as close as possible to the
headphone connector. To maximize ESD protection diode effectiveness, place diodes D1, D2, and D3 as close as
possible to the headphone connector. If capacitance larger than 1 pF is required for D1 and D2, both components
should be placed between F1, F2, and F3 and the headphone amplifier to minimize antenna shunt capacitance.
Place the chip as close as possible to the headphone connector to minimize antenna trace capacitance. Keep the
trace length short and narrow and as far above the reference plane as possible, restrict the trace to a microstrip
topology (trace routes on the top or bottom PCB layers only), minimize trace vias, and relieve ground fill on the
trace layer. Note that minimizing capacitance has the effect of maximizing characteristic impedance. It is not
necessary to design for 50  transmission lines.
4.2. Cable antenna (Si468x)
The charger cable of a consumer product can be used as an FM/FMHD/T-DMB/DAB antenna. This section
describes how to interface the Si468x VHFI input to a cable antenna.
4.2.1. Cable Antenna Design
A typical cable antenna contains multiple inner wires/conductors, which are covered with a protective ground
shield. The coupling between the wires and the shield can cause the antenna to have large capacitance in the
several hundred pF range. In order to boost the received FM/FMHD/T-DMB/DAB voltage, it is necessary to
minimize this capacitance. This reduction can be achieved by placing ferrite beads in series with each of the
antenna’s conductors.
4.2.2. Cable Antenna Schematic
Figure 6. A Typical Cable Antenna Application
To resonate the cable antenna within the FM band, the antenna’s capacitance needs to be reduced. As described
in Section 4.2.1, 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 (VHFI input) on the Si468x digital radio chip. Each of the components in the schematic above
is explained in detail below:
L1 (120 nH) is the tuning inductor. This is the typical value used to resonate the cable antenna in the center of the
DAB Band.
L2 (36 nH) is the tuning inductor. This is the typical value used to parallel with L1 to resonate the cable antenna in
the center of the DAB Band.
C13 (100 pF) is a dc blocking cap placed between the VHFI pin and the cable antenna ground.
F1(1.5 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 Si468x chip.
F2 (1.5 k at 100 MHz) is a series ferrite placed on the signal conductor in the cable antenna. Note that series
Rev. 0.2
9