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

[Marcus D. Leech]

On 05/26/2016 04:09 AM, Piotr Krysik wrote:
> Is there some good candidate that can be asked to do that? When I search
> for rtlsdr driver the first page with actual source code is osmocom's site:
>
> sdr.osmocom.org/trac/wiki/rtl-sdr
>
> Maybe they have the maintainer who feels responsible for how this code
> works?
> I can try to correct this offset in software (especially if it doesn't
> change too often) but it doesn't seem as the optimal solution. Frequency
> offset estimation might not be perfect either.
>
> --
> Piotr
Peter East has been playing with stuff as well, although his website has 
been suffering from the "Slashdot Effect(tm)" since RTL-SDR blog
  pointed to his paper a couple of days ago.  Now, he does all his 
stuff post-facto, rather than in real-time, but i don't see why there
  couldn't be two stages of 'sync' state in your code--one to do time 
synchronization, the other to do frequency-offset estimation based on
  the phase of the cross-correlation after time sync.    The residual 
frequency offset (which shows up as a phase walk) is, according to
  Peter, always some small multiple of about 0.072Hz--he measures the 
slope of the cross-correlation a few thousand samples apart, and
  uses that to estimate the frequency offset.   That works well.

Ideally, yes, you just want the hardware to behave correctly--and for 
the standard drivers to take care of this.  But doing this in your
  multi-rtl block isn't a lot of extra work, and it means you get no 
residual phase-walk whether dither is turned on or off.


> W dniu 25.05.2016 o 21:36, address@hidden pisze:
> > The AirSpy uses the R820T2 chip for the tuner, but a different
> > sampling/DSP "engine".
> >
> > Yes, making the charge-pump and "dither" mods will help with
> > phase-coherence.
> >
> > Somebody needs to "own" the rtlsdr driver, and merge in the last
> > couple of years of field experience and branching that has gone on
> > with it.
> >
> >   
> >
> >   
> >
> >   
> >
> > On 2016-05-25 15:04, Piotr Krysik wrote:
> >
> > > Hi Marcus,
> > >
> > > I don't know much about AirSpy.
> > >
> > > Does it use the same demodulator chip as current RTL-SDR dongles?
> > > And does it mean that change to low level part of rtlsdr driver might
> > > help to get rid of that frequency offset?
> > >
> > > --
> > > Piotr
> > >
> > > W dniu 25.05.2016 o 16:35, address@hidden
> > > <mailto:address@hidden> pisze:
> > > > There are a couple of issues with the rtlsdr driver used by gr-osmocom
> > > > in this regard:
> > > >
> > > >   
> > > >
> > > >    (A) The charge-pump loop current is too constrained for the higher
> > > > frequencies
> > > >
> > > >    (B) The "dither" option appears to have a bias that causes a (small)
> > > > frequency offset.
> > > >
> > > >   
> > > >
> > > > The driver that AirSpy uses fixes both of these, although without
> > > > "dither", the tuning granularity is worse.  Not sure this matters.
> > > >
> > > >   
> > > >
> > > >   
> > > >
> > > >   
> > > >
> > > > On 2016-05-25 09:28, Marcus Müller wrote:
> > > >
> > > > > That, or simply, the output clock VCO changes its reaction to the
> > > > > control voltage under certain circumstances (temperature, frequency) so
> > > > > much that the control loop loses the ability to reach stationary
> > > > > exactness (e.g. due to natural limits on the magnitude of the VCO
> > > > > voltage). These devices definitely were made with cost in mind – not
> > > > > with maximum reliability, and hence I can believe that for example with
> > > > > the Elonics E4000 tuner, the charge pump used to generate the VCO
> > > > > voltage simply might deteriorate with temperature.
> > > > >
> > > > > Cheers,
> > > > > Marcus
> > > > >
> > > > > On 25.05.2016 14:25, Sylvain Munaut wrote:
> > > > > > Hi,
> > > > > >
> > > > > > > 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 !).
> > > > > > If the phase comparator is digital ( i.e. a XOR ) and the input clock
> > > > > > is somewhat analog, the gate thresholds might vary depending on
> > > > > > temperature, thus shifting the cycle a bit.
> > > > > >
> > > > > > Just a thought.
> > > > > >
> > > > > > Cheers,
> > > > > >
> > > > > >      Sylvain
> > > > > >
> > > > > > _______________________________________________
>
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