GSM-noise "buzz" issue / ways forward

Uwe Klein wiederling at
Mon Sep 22 17:13:18 CEST 2008

On 9/22/08, Andy Green <andy at> wrote:
>  Hash: SHA1
>  Somebody in the thread at some point said:
>  | Is there any way to switch the audio path to some file output?
>  | i.e. can I (potentially) record "buzz" without an active GSM connection
>  | ( while irradiating the phone from an external HF source )
>  | would be cheaper and easier to set up that way.
>  Werner pointed out yesterday to a guy asking the same thing that
>  /usr/share/openmoko/scenarios/capturehandset.state
I'le look into that.
>  should be the thing.
>  As far as I know, nobody yet tested this with buzz and so confirmed that
>  the effective buzz problem actually comes at the MIC inputs to WM8753.
>  | <someone>
>  |>  Well maybe luckily we *might* eventually find a magic place for a
>  (33pF) C
>  |>  somewhere near Wolfson chip, to rework GTA02 out in the field this way.
>  |>  Anyway I'd rather expect this magic fix to be found by try and
>  error, rather
>  |>  than probing on a setup that's completely changed by mere applying a
>  probe to
>  |>  it.
>  |
>  | Could someone with the device open place 100Ohm resistor in series with
>  | 3/4pF from  the mic line to GND and test it? ( instead of the ferrite
>  bead.
>  |
>  | Is the trace in the top layer of the pcb or is it burried?
>  I don't have gerber layer data, but the bead was placed in series with
>  headphone jack p4, it should be possible to try.
>  But I don't have 3.. 4 pF caps lying around, I order some for tomorrow.

4pF @1.8GHz ~= 22Rx, anything in the range of 2..20pF will be OK I guess.

we can experiment a bit further when you are here.
i have a stash of 0805 parts and should be able to badger some 0204
size stuff from the regulars here.


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