ECN0008 - Mystery cap

[Werner Almesberger]

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
  designs.

- 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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[AlvieBoy]

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

Fine for me. I also made some simulations with different values and attenuation was very very similar.

>
> If nobody disagrees, then this should be the last change needed for
> ECN0008, and we can review and close that monster as well.
>
> - Werner
>
> _______________________________________________
> gta02-core mailing list
> [hidden email]
> https://lists.openmoko.org/mailman/listinfo/gta02-core

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[Werner Almesberger]

?lvaro Lopes wrote:
> Fine for me. I also made some simulations with different values and
> attenuation was very very similar.

Perfect. 22 pF it is then. I added a brief explanation to ECN0008.
We close the book on another mystery :-)

I reviewed ECN0008 and made the following small changes:

| [...] Connect R7610 to LNA_AVDD.

I think you meant R7618. Changed it accordingly. There are also a few
more components that were removed that weren't mentioned in the ECN:

| - Remove R7617 and C7623 (pull-down and buffer for VCONT)
| - Remove CON7602, C7631, and L7609 (external antenna connection)

I've moved it to "Execute". If nobody finds any further problems,
I'll close it in the next days.

- Werner

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