GSM-noise "buzz" issue / ways forward

Uwe Klein wiederling at
Mon Sep 22 14:38:22 CEST 2008

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.
The only thing you change is that reflection from an open circuit is changed
to reflection from a short. ( and you will have changed the freq. of resonance
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.

>  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..


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