GSM-noise "buzz" issue / ways forward

Andy Green andy at
Mon Sep 22 14:48:22 CEST 2008

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

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.

| The only thing you change is that reflection from an open circuit is
| to reflection from a short. ( and you will have changed the freq. of
| which could put you even further down the drain.)
| Either you get rid of the crosstalk _or_ you add _lossy_ elements to your
| transmission line.
| A resistor with about the line impedance ( or 1/2 Z  if you have access to
| the middle of the line in series with a  small cap to ground can help.
| The same goes for ferrite beads ( or flat pieces glued onto the trace )
| (the right) ferrite beads are lossy.

It sounds like you have a handle on it, but I am stuck asking myself...
line impedence... which nets are actually involved?  Headset jack p4 is
proven to be implicated but how that comes to the LF action...

|>  For both going forward and to solve issue for phones in the field, I get
|>  the feeling we can't do justice to understanding this problem internally
|>  and need to go talk to people experienced in this kind of RF issue.  Uwe
|>  and his folks sound like a first port of call.  I know a commercial
|>  outfit in UK I worked with before we can approach as well if needed.
| I plan to test a meassuring setup with my own freerunner this week.
| I'le contact Andy after that is running for meassuring his phone..

That's great, any help is mega appreciated.

- -Andy
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