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SI4734

Si47XX ANTENNA, SCHEMATIC, LAYOUT, AND DESIGN GUIDELINES

Silicon Laboratories 

AN383

The actual CTUNE needed then has to be well within the available range of CTUNE, which is:

CTUNE min = 0.25 pF

CTUNE max = 47.75 pF

CTUNE can be read from the chip and the range will be 1–191. Each number represents 0.25 pF, so the range is

0.25 to 47.75 pF.

Example 1

Conchip parasitic = 5 pF

CPCB = 4 pF

CANT = 1 pF

Ctotal parasitic capacitance = 5 + 4 + 1 = 10 pF

LTUNE = 120 nH

Desired FM frequency = 76–108 MHz

At 76 MHz > Ctotal = 37 pF, CTUNE = 37 – 10 = 27 pF

At 108 MHz > Ctotal = 18 pF, CTUNE = 18 – 10 = 8 pF

In this case at the FM frequency of interest (76 MHz to 108 MHz), the needed CTUNE range (8–27 pF) is within the

available CTUNE range on the chip (0.25–47.75 pF) which means that the 120 nH inductor value is a good choice.

Example 2

Conchip parasitic = 5 pF

CPCB = 10 pF

CANT = 1 pF

Ctotal parasitic capacitance = 5 + 10 + 1 = 16 pF

At 76 MHz > Ctotal = 37 pF, CTUNE = 37 – 16 = 21 pF

At 108 MHz > Ctotal = 18 pF, CTUNE = 18 – 16 = 2 pF

In this case at 108 MHz even though CTUNE needed (2 pF) is still higher than the min CTUNE (0.25 pF), it is not

advisable to keep LTUNE at 120 nH. Variation in the parasitic capacitance from the PCB and different components

may result in the CTUNE needed to be equal or less than 0.25 pF. It is advisable then to lower the LTUNE value to

100 nH.

Solution

Make LTUNE = 100 nH

At 76 MHz > Ctotal = 44 pF, CTUNE = 28 pF

At 108 MHz > Ctotal = 22 pF, CTUNE = 6 pF

In this case the needed CTUNE range (6–28 pF) is well within the available CTUNE (0.25 to 47.75 pF).

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Rev. 0.8

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