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Re: [Discuss-gnuradio] DBSRX coherent problem (Phase Noise)

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] DBSRX coherent problem (Phase Noise)
Date: Fri, 12 Nov 2010 08:49:37 -0800
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On 11/12/2010 08:32 AM, tom_wgs wrote:

Hi all

I am trying to create a simple direction finding device using the Ettus
USRP1 and 2 DBSRX receivers.  However I am finding a large amount of phase
noise between the two receivers.  Even across  a 128 sample collection I
find over 60 degrees of phase change.  Because the two receivers are driven
from the same 64M clock I had hoped and read that they would be coherent
over the short term.  Is there something special I need to program to get
the receivers more in sync?

I have read many of the comments about phase noise on the boards and while
there are reports I did not get the feel that the values were so high.  I
have found that using an FFT and calculating the phase at the peak provides
processing improvements, however there is still a 2 - 6 degree standard
deviation in the phase.  This calculated phase was for 200 FFTs of 128

I am using the gnuradio-3.2 software to control the usrp and  simply
collecting raw I&  Q data to a file.  I have a signal generator outputting a
tone signal at 1.88 GHz, -50dBM.  The signal goes in to a splitter and then
directly into the receivers.  I modified the software to be able to set the
frequency&  gain of both receivers.  The data is then written to a file and
I process the data using Matlab.  I am using the std_2rxhb_2tx_dig.rbf
firmware file but have also used the std_2rxhb_2tx.rbf firmware and
collected complex data with the same results.


What phase noise improvements are seen in the new DBSRX2 daughter boards?

The DBSRX2 has much better close in phase noise, so it will improve on this. However, I think what you are seeing with the original DBSRX should look better than that. Can you do an FFT of each input signal and send a link to them? I think you might see a lot more noise on one than the other, indicating a problem with one board.


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