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Somebody in the thread at some point said: | On 9/22/08, Andy Green <an...@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 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 PSRR? 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.