[gta02-core] ECN0008 - Mystery cap

Werner Almesberger werner at openmoko.org
Tue Oct 13 15:37:54 CEST 2009

Yesterday, Joerg, Rene, and I discussed the GPS antenna input some more
on IRC. The conclusion:

- Joerg remarked that all the reference circuits from u-blox and Atmel
  have a C-C-L circuit there, which strongly suggests that this is
  indeed all we need.

- Rene noticed that the ATR0610 data sheet says that the input is
  prematched, meaning that it doesn't behave just like a 50 Ohm
  "standard" input, and that we're well advised to follow the reference

- Joerg suggested a value of 15 ... 22 pF for C7601. The values I found
  in the various reference designs range from 8.6 pF to 47 pF. Rene
  calculated the impedance for a few configurations, and they all ended
  up being very similar.

- We have a 22 pF among the components removed.

So I think 22 pF is a good start. In case we get horrible results, we
can always swap it for different values.

If nobody disagrees, then this should be the last change needed for
ECN0008, and we can review and close that monster as well.

- Werner

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