GSM-noise "buzz" issue / ways forward
wiederling at googlemail.com
Mon Sep 22 14:53:22 CEST 2008
On 9/22/08, Andy Green <andy at openmoko.com> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> Somebody in the thread at some point said:
> | On 9/22/08, Andy Green <andy at openmoko.com> 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
> 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?
More information about the hardware