DBRX coherent problem (Phase noise)

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


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)


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.

What phase noise improvements are seen in the new DBSRX2 daughter

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