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AN383

Si47XX ANTENNA, SCHEMATIC, LAYOUT, AND DESIGN GUIDELINES

Silicon Laboratories 

3.2. Headphone Antenna Schematic

AN383

Figure 9. Headphone Antenna Schematic

The headphone antenna implementation requires components LMATCH, C4, F1, and F2 for a minimal

implementation. The ESD protection diodes and headphone amplifier components are system components that will

be required for proper implementation of any tuner.

Inductor LMATCH is selected to maximize the voltage gain across the FM band. LMATCH should be selected with a Q

of 15 or greater at 100 MHz and minimal dc resistance.

AC-coupling capacitor C4 is used to remove a dc offset on the FMI input. This capacitor must be chosen to be large

enough to cause negligible loss with an LNA input capacitance of 4–6 pF. The recommended value is 100 pF–1 nF.

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 FMIP such as left and right audio, microphone audio, switching, etc. In the example shown

in Figure 9, 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 resistance at 100 MHz is desirable to maximize RSHUNT,

and therefore, RP. Refer to " Appendix A—FM Receive Headphone Antenna Interface Model" on page 50 for a

complete description of RSHUNT, RP, etc.

ESD diodes D1, D2, and D3 are recommended if design requirements exceed the ESD rating of the headphone

amplifier and the Si47xx. Diodes should be chosen with no more than 1 pF parasitic capacitance, such as the

California Micro Devices CM1210. Diode capacitance should be minimized to minimize CSHUNT, and therefore, CP.

If D1 and D2 must be chosen with a capacitance greater than 1 pF, they should be placed between the ferrite

beads F1 and F2 and the headphone amplifier to minimize CSHUNT. 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.

C9 and C10 are 125 µF ac coupling capacitors required when the audio amplifier does not have a common mode

output voltage and the audio output is swinging above and below ground.

Optional bleed resistors R5 and R6 may be desirable to discharge the ac-coupling capacitors when the headphone

cable is removed.

Rev. 0.8

19

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