GSM-noise "buzz" issue

Werner Almesberger werner at
Sat Sep 20 22:13:29 CEST 2008

Joerg Reisenweber wrote:
> See placement of R4408. I think that's enough of a good story.

Hmm, I'm beginning to understand your MICBIAS theory. Both C3014
and C3015 invitingly offer their MICBIAS end to any RF coming out
of the R4401 and R4408 pair.

Luckily, the other capacitors in the area only show their GND end
to this probable RF source.

No amount of component removal besides JK4401 could fix that, so
that's consistent with experiment results.

A more specific test for MICBIAS contamination would be to remove
R4305 and to bias the internal microphone from an external source
(through a shielded cable ;-), e.g., across D4301, which is easily
accessible. Has something like this been tried ?

It's a pity that you don't have a proper RF generator. All those
tests with the GSM module or an external phone as the RF source
just introduce so many parameters completely outside your control
that it's almost impossible to get meaningful results.

I wonder how hard it could be to build a little 1W RF generator
that's tunable in the 1-2GHz range.

Anyway, I disgress. What really confuses me is that we still don't
seem to have an explanation for why shorting MIC to GND doesn't
seem to help. Or do we ? Or does it in the end ? :-)

By the way, there are two goals here:

1) Make sure we don't repeat the same mistake in the future. I
   agree that you're perfectly right with just keeping RF out of
   the can.

2) Find a work-around for the GTA02 units already in the field.

My questions are more pointed at the latter than the former.

- Werner

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