GSM-noise "buzz" issue

Andy Green andy at
Mon Sep 22 11:46:31 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:

|> You would think it also depends on output power from our RF section; it
|> might be worth trying with "piece of wire" antenna to force it to use
|> highest transmit power if otherwise your base station is close.
| Using "piece of wire" might serve well to spoil SWR of transmitter, thus
| reducing antenna gain. So it's correct BTS will advice MT to increase

I dunno if that is the case or not, but I do know the GSM system on the
board will act differently at power using $WIRE rather than the intended
antenna, and we can get a clue about what goes on by rattling as many
doorhandles as possible.  It certainly means something that the buzz
still comes happily or put another way we would be very interested if
the buzz stopped coming with $WIRE.

|> If it isn't that you are using low power, that's pretty interesting, it
|> couples at 1.7GHz but not 1.8 - 1.9GHz then.  It seems something we are
|> missing is frequency response of the radiated coupling to the victim net.
| That's why I asked for it some mails ago, so we had an idea which
| are the ones GTA02 is sensitive to.
| Though the result wouldn't probably change anything about proposed
fix, which
| is to place beads/EMI-filters there on JK4401, and please do it as
soon as
| possible.

I looked at this last night, but my setup is too crap to get a sensible
result.  All I have around is some old scope probes terminating in BNC,
so I cut one off and used that cable from the tracking generator (it's
like 2V p-p) first for radiated coupling I placed the insulated core up
the inside of the headset jack and looked on MICBIAS with the spectrum
analyser via another scope probe, then secondly I placed the tracking
generator output directly on pin 4 of the jack and looked at MICBIAS and
surrounding components.

But presumably because these scope probes and cables were only rated for
100MHz, just kinking the wire or moving the leads around totally changed
the response far more than what I was measuring.  I dunno if I should
terminate the lead to 50R, via a cap, no idea how to improve it at the
moment.  I think it will be very interesting to meet Uwe and his guys.

- -Andy
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