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SI4706

Si47XX ANTENNA, SCHEMATIC, LAYOUT, AND DESIGN GUIDELINES

Silicon Laboratories 

Antenna,PCB & LNA

Match

AN383

CP RP

LMATCH

Figure 33. Parallel Resonant RLC Circuit Model

CP = Parallel capacitance

RP = Parallel resistance

LMATCH = Inductance Match

LMATCH is required to prevent the antenna from being shorted to ground at RF frequencies and to provide a path to

ground at audio frequencies for return current from the headphone amplifier. Selecting the proper value of LMATCH

will maximize voltage gain across the FM band for optimal RF performance.

To maximize voltage gain across the FM band:

1. The value of RP should be maximized to maximize the voltage at the LNA input.

2. The Q of the circuit should be minimized to maintain a flat response across the FM band.

3. The value of LMATCH should be chosen such that the circuit resonates in the center of the FM band.

The value of RP should be maximized to maximize the voltage at the LNA input. The parallel resistance, RP, shown

in Figure 33, “Parallel Resonant RLC Circuit Model” is defined as:

RP = RLNA  RSHUNT  RANT*

The LNA resistance, RLNA, will range from 4 to 6 k during normal operation. The shunt resistance, RSHUNT, is the

parallel addition of ferrite resistance on the left and right audio conductors, and other conductors for microphone

audio, switching or other circuits, if applicable. RSHUNT should be as large as possible to maximize RP. Specific

recommendations for ferrite values can be found in Section "3.2. Headphone Antenna Schematic" on page 19. The

antenna source resistance, RANT, will range from approximately 500  for shorter antennas to several thousand

ohms for longer antennas. RANT* is the parallel circuit model for RANT near the resonant frequency, f, of the RLC

circuit, and is approximated as:

RANT*  RANTQANT2 + 1

=

RANT





2--------f--R----A----N1---T---C-----A---N----T-

2

+

1

The antenna length should be 1.1 to 1.45 m, with optimal performance at 1.45 m to maximize RANT*.

The Q of the parallel resonant RLC circuit shown in Figure 33, “Parallel Resonant RLC Circuit Model” is defined as:

QP

=

---R----P----

C-L----PP-

Rev. 0.8

51

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