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

[Juha Vierinen]

I was using a dongle with r820.

juha

> On May 25, 2016, at 23:27, Piotr Krysik <address@hidden> wrote:
> 
> 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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>>    <mailto:address@hidden <mailto:address@hidden>>
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>>> 
>>> 
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>> 
>> 
>>    --
>>    JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de
>>    l'Epitaphe, 25000 Besancon, France
>