In my testing, this phase rate difference seemed constant and I could simply calibrate it out by looking at the phase rate term estimated from the phase of the cross-correlated noise. The samples stay aligned for hours, so the issue is caused by the tuner or the DDC. One theory that I had was that the multi-rtl driver somehow sets up the dongles differently, but I never got around to looking at the code.
I think this was the script I used to figure out what the phase drift was: https://github.com/jvierine/chirpsounder/blob/master/apps/passive_radar/rnoise.py Here's the IQ plot to prove that the cross correlated phase is stable over 6000 seconds: http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html juha On Wed, May 25, 2016 at 12:14 PM, <jean-michel.fri...@femto-st.fr> wrote: > From our own experience with dual-dongle measurements, the phase drift > seems to be strongly related to R820T(2) temperature. We reduced > significantly > the phase drift by gluing a large heat sink common to both chips on both > dongles, without completely removing this effect (we aim at measurements > lasting > multiple hours). At the moment the only option we could think of (mail to > this > mailing list dated 28 May 2015) is switching to a reference clock to > calibrate > for the phase difference between the local oscillators, but the actual > cause > of the drift remains a mistery (probably due to the implementation of the > PLL, > but I cannot understand why Phase Locked Loops would drift in Phase !). > > JM > > > It wouldn't be as simple as it was for me as a developer and as it > > (hopefully) is for the end user without your hardware mod. > > > > Can you say something more about the residual center frequency > > difference? Where might it come from? I prepared little test of > > coherency between the receivers (multi-rtl/examples/test_multirtl.m). > > Among all the figures that it shows there is a plot of relative phase > > offset of signals coming from the receivers. In fact I have seen linear > > phase change on that plot - that corresponds to some central frequency > > offset. If I know what is the source of this offset maybe I will be able > > to find some way to fix it in software. > > > > -- > > Piotr > > > > W dniu 25.05.2016 o 08:24, Juha Vierinen pisze: > > > This is awesome! I'll definitely try this out soon. I use one off > > > python scripts to find the sample offset and the small residual center > > > frequency difference. This simplifies the process significantly. > > > > > > This should make it much easier to implement a passive radar block, or > > > an interferometry block. > > > > > > juha > > > > > > On Tue, May 24, 2016 at 4:03 PM, Piotr Krysik <per...@o2.pl > > > <mailto:per...@o2.pl>> wrote: > > > > > > Hi all, > > > > > > I want to announce new GNU Radio related project prepared by me - > > > Multi-rtl: > > > https://github.com/ptrkrysik/multi-rtl > > > > > > It is a Gnu Radio block that combines multiple RTL-SDR receivers > into > > > one multi-channel receiver. > > > > > > Only hardware modification to RTL-SDR dongles required is > connecting > > > them to a common clock source (i.e. one of the dongles' oscillator > as > > > Juha Verinen showed once). Each channel can work on a different > > > central > > > frequency. > > > > > > Everyone who wants to know how it was achieved is invited to read > my > > > github page: > > > > > > https://ptrkrysik.github.io > > > > > > Best Regards, > > > Piotr Krysik > > > > > > _______________________________________________ > > > Discuss-gnuradio mailing list > > > Discuss-gnuradio@gnu.org <mailto:Discuss-gnuradio@gnu.org> > > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > > > > > > > > > _______________________________________________ > > Discuss-gnuradio mailing list > > Discuss-gnuradio@gnu.org > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > -- > JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 > Besancon, France >
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