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 -- View this message in context: http://old.nabble.com/DBSRX-coherent-problem-%28Phase-Noise%29-tp30200932p30200932.html Sent from the GnuRadio mailing list archive at Nabble.com. _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
