BT mystery filter - the truth about C1

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BT mystery filter - the truth about C1

Werner Almesberger
(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
http://www.darfon.com.tw/English/MLCC_OrderingInfo.asp
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.

- Werner

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Re: BT mystery filter - the truth about C1

Rene Harder
How do we proceed here, just change the value in our schematics and
place a note or is it necessary to write a ECN for this?

Rene

Werner Almesberger wrote:

> (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
> http://www.darfon.com.tw/English/MLCC_OrderingInfo.asp
> 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.
>
> - Werner
>  


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Re: BT mystery filter - the truth about C1

Werner Almesberger
Rene Harder wrote:
> How do we proceed here, just change the value in our schematics and
> place a note or is it necessary to write a ECN for this?

I don't think we need an ECN for this. It's not actually a change,
just an inconsistency in Openmoko's schematics that we're cleaning up.
It's good to put a note in our schematics, though, so reviewers won't
trip over it.

- Werner

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