Re: FM Carrier Recovery for AOA Calculation

[Marcus Müller]

Always wondered what the state-of-the-art is in oscillator drift compensation; I bet 
there's more to it than estimating a second derivative of one LO's phase against the other 
and throwing Kalman at it until it doesn't move anymore, or whether there's better models...

but you're right. If this is more than "a one-shot measurement immediately after 
calibration", you'll need to share a reference clock; even an extremely nice frequency 
error of $2\cdot 10^{-8}$ between the two receivers would, at 1 GHz carrier, mean that the 
phase of one rotates away from the other by $40 \pi$ every second, so you have like 10 ms 
until you lose any sensible idea of direction. I don't know how well the calibration 
routine is able to compensate such small frequency errors, it might actually be OK for 
longer, or it might not.

Best regards,
Marcus

On 01.09.23 22:48, Marcus D. Leech wrote:

> On 01/09/2023 16:39, Marcus Müller wrote:
> > Ah, nice! yeah, then this would work – but! any oscillator has drift, so two local 
> > oscillators will not stay on the same phase for long; if your first couple measurements 
> > are correct, but suddenly your directions become very wrong, that's what I'd investigate.
> >
> > Cheers,
> > Marcus
> Looks like the BB60C supports an external 10MHz reference clock, which would be utterly 
> mandatory for anything requiring
>   mutual phase-coherence, whether that's *sufficient* depends very very much on the 
> internal architecture of the BB60C.
>
>
> > On 01.09.23 19:16, Michael Berman wrote:
> > > The gr-aoa module has a calibration phase where you connect a source with a splitter 
> > > and roughly equally lengthened cables to the two sources and it cross-correlates the 
> > > two inputs to determine an initial phase difference.  Then, while the script is still 
> > > running, you disconnect the calibration source and connect the antenna's, again with 
> > > roughly equally lengthened cables to run the MUSIC algorithm.
> > >
> > >
> > > Thank you very much,
> > >
> > > Michael Berman
> > >
> > > On Fri, Sep 1, 2023 at 11:00 AM Marcus Müller <marcus.mueller@ettus.com 
> > > <mailto:marcus.mueller@ettus.com>> wrote:
> > >
> > >     If you don't know the relative phase of your two receiver chains, how are you 
> > > going to
> > >     know the direction of a signal?
> > >
> > >     On 01.09.23 17:37, Michael Berman wrote:
> > > >     Marcus,
> > > >
> > > >     Thanks for the reply!  I apologize, the GR package is gr-aoa, not gr-music, 
> > > > and can
> > > >     be found here (https://github.com/MarcinWachowiak/gr-aoa
> > > >     <https://github.com/MarcinWachowiak/gr-aoa>). The NOAA broadcast is a NBFM 
> > > > signal.     I am using 2 Signal Hound receivers (https://signalhound.com/products/bb60c/
> > > >     <https://signalhound.com/products/bb60c/>) with 2 antennas spaced on a 
> > > > beam.  Do I
> > > >     need to synchronize the 2 receivers with an external clock, or should they 
> > > > be fine
> > > >     free running independently?
> > > >
> > > >
> > > >     Thank you very much,
> > > >
> > > >     Michael Berman
> > > >
> > > >     On Fri, Sep 1, 2023 at 9:25 AM Marcus Müller <mmueller@gnuradio.org
> > > >     <mailto:mmueller@gnuradio.org>> wrote:
> > > >
> > > >         Not familiar with the details of NOAA signalling, but isn't the carrier 
> > > > of an FM
> > > >         signal
> > > >         *the FM signal*?
> > > >
> > > >         For a DoA estimate, you'd correlate the different receive chains with 
> > > > each other
> > > >         to get a
> > > >         phase; so, as long as the signals do have some bandwidth that makes the 
> > > > problem
> > > >         less
> > > >         ambiguous, it'd work with any signal. I'm sadly not familiar with 
> > > > gr-music (and
> > > >         can't find
> > > >         it on cgran.org <http://cgran.org>), but MUSIC works as long as the 
> > > > signals at
> > > >         the different receive antennas
> > > >         are correlated and noise is not. You do not have to preprocess your FM 
> > > > signal!
> > > >
> > > >         Best,
> > > >         Marcus
> > > >
> > > >         On 01.09.23 17:09, Michael Berman wrote:
> > > >         > Does anybody know if there is a way to recover a carrier of an FM 
> > > > signal to
> > > >         use for an
> > > >         > Angle of Arrival calculation?  I am using GNURadio and gr-music and I 
> > > > am
> > > >         trying to use the
> > > >         > NOAA Weather Radio signals.
> > > >         >
> > > >         > Thank you very much,
> > > >         >
> > > >         > Michael Berman