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AN383

Si47XX ANTENNA, SCHEMATIC, LAYOUT, AND DESIGN GUIDELINES

Silicon Laboratories 

AN383

3.4. Headphone Antenna Layout

To minimize inductive and capacitive coupling, inductor LMATCH and headphone jack J24 should be placed together

and as far from noise sources such as clocks and digital circuits as possible. LMATCH should be placed near the

headphone connector to keep audio currents away from the Si47xx.

To minimize CSHUNT and CP, 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 FB1 and FB2 and the headphone amplifier to minimize CSHUNT.

Place the chip as close as possible to the headphone connector to minimize antenna trace capacitance,

CPCBANT. 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.

To reduce the level of digital noise passed to the antenna, RF shunt capacitors C5 and C6 may be placed on the

left and right audio traces close to the headphone amplifier audio output pins. The recommended value is 100 pF

or greater, however, the designer should confirm that the headphone amplifier is capable of driving the selected

shunt capacitance.

3.5. Headphone Antenna Design Checklist

 Select an antenna length of 1.1 to 1.45 m.

 Select matching inductor LMATCH to maximize signal strength across the FM band.

 Select matching inductor LMATCH with a Q of 15 or greater at 100 MHz and minimal dc resistance.

 Place inductor LMATCH and headphone connector together and as far from potential noise sources as possible

to reduce capacitive and inductive coupling.

 Place the Si47xx close to the headphone connector to minimize antenna trace length. Minimizing trace length

reduces CP and the possibility for inductive and capacitive coupling into the antenna by noise sources. This

recommendation must be followed for optimal device performance.

 Select ferrite beads F1–F2 with 2.5 k or greater resistance at 100 MHz to maximize RSHUNT and, therefore,

RP.

 Place ferrite beads F1-F2 close to the headphone connector.

 Select ESD diodes D1-D3 with minimum capacitance.

 Place ESD diodes D1-D3 as close as possible to the headphone connector for maximum effectiveness.

 Place optional RF shunt capacitors near the headphone amplifier’s left and right audio output pins to reduce the

level of digital noise passed to the antenna.

Rev. 0.8

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