GSM-noise "buzz" issue / ways forward
wiederling at googlemail.com
Mon Sep 22 14:38:22 CEST 2008
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 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
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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