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

[tom_wgs]

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
points. 

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.

http://old.nabble.com/file/p30200932/Diagram.jpg 

The frequency 1.88 GHz
Decimation 128
The sample rate is 500k/sec 

The first image is the phase between RXA & RXB for each sample of a 128
sample collection.  
Phase is calculate using time domain complex values.  
phase = RXB / conjugate(RXA)

http://old.nabble.com/file/p30200932/Phase_accross_128_samples.jpg 

The sample changes over 60 degrees across this very short sample period
The data sample can be anywhere in a 10 second collection and the results
are similar.



The second image shows the plot of the two real components of the same 128
samples.  
The phase shift is so large it is visible in the plot.  I
I noticed that the tuned frequency is about 12 kHz off target, but this is
within the expected spec. of the system.


http://old.nabble.com/file/p30200932/Real-RXa_Real_RXb.jpg 


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

Thank you for your time and any help would be very much appreciated.
Tom
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