Thanks for the informative post. I've been searching for a way to turn off the dithering.
The I/Q plot show complex cross-correlation between two noise signals at 100 MHz center frequency (2.4 MHz bandwidth noise). The phase of the signal can be converted into time and Allan deviation, but I never did. I was just happy to see a straight line. juha On Fri, Jul 7, 2017 at 08:05 jmfriedt <jean-michel.fri...@femto-st.fr> wrote: > I am tackling again the coherence issue with multi-RTL dongles for > interferometric measurements. > I have two R820T2 based DVB-T dongles connected to the same USB bus (USB > hub) and am feeding > both dongles with a high quality 100-MHz source (-34 dBm output of a > Rohde&Schwarz SML03 synthesizer, > 30 dB gain on the RF frontend of the dongles) -- setup shown at > http://jmfriedt.sequanux.org/DSC06032.JPG. > > I am trying to reproduce the graph "IQ plot of the z_1/z_2 relative phase > signal over ~6000 seconds at > 1 Hz sample rate" from > http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html > Actually I notice that this graph shows something in units in the tens of > thousands: what is > "Z1/Z2 relative phase" ? How can I convert this axis to angle in degrees ? > > I have manually applied the patch described at > http://www.rtl-sdr.com/forum/viewtopic.php?f=1&t=531 to > turn off dithering (I have checked that the function is called when > starting gr-osmosdr -- whether > dithering is actually disabled is challenging to assess). The resulting > phase measurement -- recording > either with multi-rtl source or two osmosdr sources, multiplying one > output by the complex conjugate > of the other, complex to phase and saving the resulting phase at 200 Hz > rate, is at > http://jmfriedt.sequanux.org/allan.pdf (left is time domain plot, right > is Allan deviation). On the > short term we have sub-degree stability, on the long term several degree > drift. I have checked that > thermal transfer between the two dongles is of utmost importance: if I > cool one of the dongles and keep > the other at room temperature, I observe phase rotations of several tens > of degrees. I believe such > long term phase instability is of little concern for passive radar > measurements where coherence over > several ms only is needed, but I find conflicting conclusions on long term > phase stability. > > Can other experimenters on this topic comment and assess whether > http://jmfriedt.sequanux.org/allan.pdf > can be improved, or if is in line with the expected behaviour of these > dongles ? > > Thanks, Jean-Michel > > -- > JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 > Besancon, France > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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