SI4721 datasheet(32/69 Pages) SILABS

32 / 69 page

AN383

Rev. 0.8

After choosing the right L1 value, the user is still required to check the on-chip varactor value READANTCAP by

sending the TX_TUNE_STATUS or FM_TUNE_STATUS command to make sure that the on-chip varactor has not

gone out-of-range. User also can jump to this second step right away and skip the first step of approximating L1 by

trying different values of L1.

The procedure to monitor the READANTCAP is as follows:



Select at least three frequencies in the bottom, middle and top of the FM band (e.g. 88, 98, and 108 MHz) and

get the READANTCAP values.



It is even better if user sweeps the entire FM band and gets the READANTCAP values.



L1 has a correct inductance value if



1 < READANTCAP < 191



READANTCAP at 88 MHz > READANTCAP at 98 MHz > READANTCAP at 108 MHz



Be suspicious when READANTCAP returns the middle value of 97. It may be an indication that the inductor value is not

correct.



L1 is not the right value and needs to be changed if either one of these three conditions occur:



READANTCAP returns the bottom value of 1: it indicates that there is too much capacitance at the TXO/LPI pin. L1 needs

to be adjusted to a smaller value or the better solution is to try to reduce the parasitic capacitance at the TXO/LPI pin



READANTCAP returns a middle value of 97: it indicates that there is way too much capacitance at the TXO/LPI pin. L1

needs to be adjusted to a smaller value or the better solution is to try to reduce the parasitic capacitance at the TXO/LPI



READANTCAP returns the top value of 191: it indicates that there is too little of a capacitance at the TXO/LPI pin (unlikely

to happen). L1 needs to be adjusted to a bigger value.

The rest of the components in the embedded antenna schematic are optional.

D1 is the ESD diode and it is only necessary when there is an exposed pad going to the TXO/LPI pin.

R1 is the ESD current limiting resistor and used in conjunction with D1. It is only necessary when there is an

exposed pad going to the TXO/LPI pin.

Note:

When using an electrically short monopole antenna for Transmit, the radiated power varies with frequency. Specifically,

as the transmit frequency increases, the monopole antenna becomes more efficient and hence the radiated power

increases. This is important when testing for FCC limit because the transmit power level of the chip needs to be adjusted

across the FM band.

5.3. Embedded Antenna Bill of Materials

Table 14. Embedded Antenna Bill of Materials

Designator

Description

Note

Tuning inductor (typically 120 nH)

Current limiting resistor for ESD, 2



Optional, only needed if there is any

exposed pad going to the TXO/LPI pin.

ESD diode

Optional, only needed if there is any

exposed pad going to the TXO/LPI pin.

SI4721 Datasheet(PDF) 32 Page - Silicon Laboratories

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