[gta02-core] BT mystery filter - the truth about C1
werner at openmoko.org
Tue Aug 4 00:06:43 CEST 2009
(For the list: Rene found that the LC filter on the BT module was way
off. The Openmoko schematics show it as C1 = 22 pF and L1 = 3.9 nH.)
I looked it up in the BOM and found a rather unexpected
MO-CAP 0.2pF 50V 0.25pF SMT0603 NPO C1608NP0208CGT DARFON
A 200 femto-Farad capacitor, with a +/- 125% tolerance ? That can't
be right, can it ? Well, if
can be trusted, that's indeed what it is.
Assuming 1 pF of parasitic capacitance, that would put us at 2.3 GHz
and 57 Ohm, not too far from where we want to be.
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