Re: [Discuss-gnuradio] Multi-rtl - making multi-channel receiver out of multiple RTL-SDR dongles

[Piotr Krysik]

Juha,

What type of demodulator did you have in the dongles used for the test?

--
Piotr

W dniu 25.05.2016 o 14:46, Juha Vierinen pisze:
> 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, <address@hidden
> <mailto:address@hidden>> 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 <address@hidden
>     <mailto:address@hidden>
>     > > <mailto:address@hidden <mailto:address@hidden>>> 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
>     > >
>     > >     _______________________________________________
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>     > >
>     > >
>     >
>     >
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>
>     --
>     JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de
>     l'Epitaphe, 25000 Besancon, France
>
>