GSM-noise "buzz" issue / ways forward

Uwe Klein wiederling at
Mon Sep 22 14:53:22 CEST 2008

On 9/22/08, Andy Green <andy at> wrote:
>  Hash: SHA1
>  Somebody in the thread at some point said:
>  | On 9/22/08, Andy Green <andy at> wrote:
>  |
>  |>  I agree it's not clear 100uF there is going to do anything good about
>  |>  the problem, unless the dips are introduced directly on MICBIAS then
>  |>  trying to smooth them there is the wrong place since they have already
>  |>  been amplified on to there and we fight the blameless amplifier.  But
>  |>  actually, I didn't see we proved quite where the buzz starts from as LF
>  |>  so we are not in a position to write this off as insane.
>  |
>  | If you have problems with  HF "infestation" a big cap will usually not
>  help.
>  Completely agree, but MICBIAS gets something different from HF during
>  the buzz behaviour, it gets small depressions (like 100mV IIRC) in it at
>  ~200Hz period.  Basically it is exactly like the PSRR was very low and
>  the impact of GSM TX action on the battery rail (also 200Hz "dents" from
>  high current draw at TX time) was showing up on MICBIAS.
>  MICBIAS as it sounds provides DC to power the microphone, now it has
>  200Hz noise in audio range we assume this forms the basis of the "buzz".
>  Someone has seen that with a 'scope and had the idea to put a big cap on
>  it to iron out the 200Hz dents.
>  What I don't understand is how we get from 1.7GHz "infestation", which
>  is clearly an issue since a bead on p4 of the headset jack does
>  something, to these LF "dents" on MICBIAS.

Do we have a sound recording of this buzz?


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