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[Discuss-gnuradio] gsm and usrp2

From: Hanno Jung
Subject: [Discuss-gnuradio] gsm and usrp2
Date: Tue, 19 Jan 2010 10:43:50 +0100

Hi everybody I'm trying to find a frequency correction burst of gsm.
This Burst is due to GMSK Modulation 67,7 kHz obove the carrier and unmodulated.

I've got an older Nokia cell phone with some monitoring software where I can read out the arfcns the phone sees and thus the frequencies of the BCCH.

The phone sees a BCCH at 940MHz but if I use the usrp2/usrp1 and the Constellation Sink in GRC I can't see the phase shift of pi/2. But if I use the FFT sink I can see the BCCH, so it has to be there or am I mistaken?
The thing is if I use as file source the captured data from
it works fine and I can see thephase shift of pi/2.
I know they're doing some processing before but it should be possible with my configuration too.

Setting is as follows:

usrp2->multiply(1000000)->constellation sink

gnuradio 3.3

30 db LNA in front of the usrp2


gain:20 db

Constellation Sink:

everything standard except
symbolrate: 270800 (gsm bitrate)

Hopefully somebody can help this is strange

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