GSM-noise "buzz" issue

[Joerg Reisenweber]

Am Fr  31. Oktober 2008 schrieb Angus Ainslie:
> On Thu, Oct 30, 2008 at 8:18 PM, Andy Green <andy at openmoko.com> wrote:
> 
> > Angus, I talked to Candy here in .tw the last few days about this, she
> > actually did the work to perform the tests Joerg is talking about.
> >
> 
> Did she use a 100uF cap ?
> 
> 
> >
> > One thing you can try that is quite simple and easy is to override
> > MICBIAS net by a ~1.5V battery on it to 0V.  She told that this alone
> > removes the buzz, it would be good to know if you can reproduce.
> >
> >
> That probably would work. I don't think I'm going to try it as I have had
> some results with the cap fix.
> 
> Initially I tried using the 100uF cap that was suggested. It kind of worked.
> there was no buzz but there was also almost no gain from the mic.

You HAVE to replace R4303 0R with a 2k2 !! (check R from mic housing MIC4301:2 
to GND! Has to be 2k2, will read 0R usually)
Otherwise this cap fix CAN NOT work. Refer to schematics, please.

> Voice 
> wouldn't get transmitted but blowing on the mic would. I removed the 100uF
> cap and then used a 1uF cap. Ther buzz was gone, a slight hum still remains
> that might have been coupling from one of the other devices in the room. I'm
> going to try in a less signal saturated environment later.
> 
> I think I'd also like to try it with MICBIAS raised to the 0.9 multiplier.

Probably won't change that much - interesting to hear of your results 
nonetheless.

> Is that done in the bootloader or in kernel space ? Could someone generate a
> an updated kernel with the higher mic bias ?
> 
> Prep the ground plane for the 100uF cap. The to photos are the same the
> smaller one is just for bandwidth reduced diets.
> 
> http://handheldshell.com/DSCN0386_small.JPG
> http://handheldshell.com/DSCN0386.JPG
> 
> Cap in place
> 
> http://handheldshell.com/DSCN0387_small.JPG
> http://handheldshell.com/DSCN0387.JPG
> 
> Jumper wire attached

You also could have used the empty pad more near to the left side (R4304)
Wire should be kept as short as possible (to reduce "antenna")

> 
> http://handheldshell.com/DSCN0388_small.JPG
> http://handheldshell.com/DSCN0388.JPG
> 
> With the 1uF cap.
> 
> http://handheldshell.com/pa310077_small.jpg
> http://handheldshell.com/pa310077.jpg
> 
> Thanks
> Angus

cheers
jOERG
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