GSM-noise "buzz" issue

[Joerg Reisenweber]

Am Mo  29. September 2008 schrieb Andy Green:
> Somebody in the thread at some point said:
> 
> | Got around to try it just today.
> | The problem for me with testing is that I (too) have a pretty good
> | signal where I life (Actually orignating about 300m away ;) ), so that
> | if there is buzz it is relatively silent.
> | To reproduce it reliably I just went into the cellar and did a longer
> | test call. Interestingly the buzz was sometimes pretty loud and
> | sometimes hardly present which did not seem to coincide with the signal
> | strength. Actually once I noticed the buzz getting louder when the
> | signal got stronger oO. What I annoyingly had all the time was a pretty
> | loud echo though :(.
> 
> I know what you mean about it coming and going, seemingly disconnected
> from what you are doing to the phone.
> 
> I was looking at the VB_SYS rail during the buzz time a couple of months
> ago, it seemed to me the buzz came and went according to what I saw on
> VB_SYS.  But this was an A5 revision.

I will include this to my mail signature:
Buzz obviously is related to TRANSMIT-power of mobile. Transmit power is 
remote controlled by the base station, so the signal BS is receiving meets 
BS' "taste". There is *no* direct relation between RECEIVE-signal strength as 
reported by signal meter on mobile, and the way BS decides to set he TX-power 
of mobile to.
Obvious example: when very remote of a strong BS, you might see good signal 
strength, but transmitter of mobile has to power up to the limit. When close 
to a BS of a small cell (usual situation in urban areas), you might see weak 
signal strength, but nevertheless mobile needs low tx-power to reach the 
nearby BS.
Then when BS sees some bad noise spoiling the signal from your phone, BS will 
decide to level up TX-power of your phone, and buzz increases without any 
change at all in your test setup.
That's also the reason why replacing original antenna with a "piece of wire" 
doesn't really change buzz situation: BS is leveling TX-power so 
the "reduced" TX-RF-field is attenuated to be same as before, though 
transmitter-amp is burning more power to achieve this identical TX-strength - 
so this "wire-antenna-test" actually is a proof for OTA-injection of buzz, as 
it has to yield *more* buzz if it was by internal coupling via RF or DC.

It's difficult to test in real live, you absolutely can't control the setup.



> 
> | As for the shorted plug: If there is buzz it amplifies it a tiny bit, if
> | there is no buzz it doesn't seem to induce buzz, but it definitely does
> | not seem to make things better.
> 
> At high frequencies "shorts" with some loop area don't necessarily have
> that low an impedence.
> 
> What did you actually short to what, everything to the 0V pin really?

The most easy way to short it is to use a copper wire of correct diameter and 
insert that instead of a plug (don't push it in too far ;-) ).
I don't expect this will yield a good result, as our tests to short 
JK4401:pin6 to GND internally also didn't help. Be aware: this isn't exactly 
GND plane, but in fact one half of the antenna dipole. RF isn't only at 
antenna, but spreads all over the device electrical surface.

cheers
jOERG
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