SI4785 データシートの表示（PDF） - Silicon Laboratories

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SI4785

Si47XX ANTENNA, SCHEMATIC, LAYOUT, AND DESIGN GUIDELINES

Silicon Laboratories 

Length (cm)

10

11

12

13

14

15

Cant (pF)

1.68

1.85

2.02

2.19

2.36

2.53

5.2. Embedded Antenna Schematic

Figure 19 shows the embedded antenna schematic.

AN383

Figure 19. Embedded Antenna Schematic

L1 is the tuning inductor and typical value is 120nH. L1 needs to be chosen such that the resonant circuit of L1 and

the total capacitance at the TXO/LPI pin will resonate at the FM band (76-108MHz). The total capacitance at the

TXO/LPI pin includes the internal on-chip varactor, the antenna and all of the parasitic capacitance at that node.

Therefore it is important to check the READANTCAP value as described below when the product is in fully

assembled configuration.

There are two steps to select the value of L1: calculate L1 using the formula below, and then monitor the on-chip

varactor value READANTCAP by sending the TX_TUNE_STATUS or FM_TUNE_STATUS command to make sure

the on-chip varactor is still in-range.

The first step is approximating the value of L1 (which can be skipped). The formula to calculate L1 is as follows:

L1 = ---2-------f-----21--C-----t-o----t-a----l

where:

f = frequency of FM band

Ctotal = total capacitance at the TXO/LPI pin

Note that Ctotal will vary because of the automatic tuning of the internal on-chip varactor. The internal on-chip

varactor has value from 1-191 for 0.25pF step which equals to 0.25pF to 47.75pF. Knowing the other capacitance

at the TXO/LPI pin which is layout and component dependent will give a range of Ctotal. After the approximate

Ctotal has been calculated, L1 can be chosen to satisfy the formula above for the FM band.

Rev. 0.8

31

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