AN851 Datasheet, PDF (7/25 Pages) Silicon Laboratories – Si468X SCHEMATICS AND LAYOUT GUIDE

4.1.2. Headphone Antenna Schematic
J1
HP_JACK
S
R
T
D1
D2
D3
AN851
L2
L1
VHFI
C13
VHFSW
F1
RIGHT CHANNEL
F2
LEFT CHANNEL
Figure 4. Headphone Antenna Design
The headphone antenna implementation requires components L1, L2, C13, F1, F2, and F3 for a minimal
implementation. In Figure 4, a headphone and circuit with headphone audio common grounded is used. The ESD
protection diodes and headphone amplifier components are system components that will be required for proper
implementation of any tuner. Inductors 1 and 2 are selected to maximize the voltage gain across the DAB/T-DMB
band. C13 is switched in with VFHSW to ensure the FM/FMHD band is properly resonated. The user should refer
to property “0x1712” of AN649 to understand how to enable and disable the switch.
Ferrite beads F1 and F2 provide a low-impedance audio path and high-impedance RF path between the
headphone amplifier and the headphone. Ferrite beads should be placed on each antenna conductor connected to
nodes other than the VHFI such as left and right audio, microphone audio, switching, etc. In the example shown in
the figure above, these nodes are the left and right audio conductors. Ferrite beads should be 2.5 k or greater at
100 MHz, such as the Murata BLM18BD252SN1. High impedance is desirable to reduce antenna coupling to the
other conductors.
L1 and L2 are used as audio ground. Diodes should be chosen with no more than 1 pF parasitic capacitance, and
diode capacitance should be minimized. If D1 and D2 must be chosen with a capacitance greater than 1 pF, they
should be placed between the ferrite beads and the headphone amplifier to minimize PCB parasitic capacitance.
This placement will, however, reduce the effectiveness of the ESD protection devices. Diode D3 may not be
relocated and must therefore have a capacitance less than 1 pF. Note that each diode package contains two
devices to protect against positive and negative polarity ESD events.
Figure 5. Optional RF Shunt Capacitors to Reduce Noise Coupling to Antenna
Rev. 0.2
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