Re: Phase Synchronize 2 USRP N200 w/ SBX cards

[jmfriedt]

For what it is worth, slide 10 of 
https://archive.fosdem.org/2020/schedule/event/fsr_software_defined_radio_based_scientific_instrumentation/attachments/slides/3895/export/events/attachments/fsr_software_defined_radio_based_scientific_instrumentation/slides/3895/fosdem2020_instrumentation.pdf
shows how phase between two inputs of a B210 is measured as the ratio
of the FFT (Aexp(jwt+phi1)/Aexp(jwt+phi2)~exp(j(phi1-phi2) even if a LO
offset w remains, here with respect to an exteral acousto-optical
modulator oscillator) and I have hidden from that discussion the need to
heterodyne digitally the signal using a Xlating FIR for weak signals as
shown on slide 24 of http://jmfriedt.free.fr/ifcs2021_tutorial.pdf
That would be most convenient for CW signals as illustrated on slide 17 
of that same presentation, otherwise phase of the cross correlation
indeed for broadband signals (slide 10 only shows the magnitude but
replace Complex to Mag with Complex to Arg),

JM

> On 06/13/2021 04:02 PM, Skyvalakis Konstantinos wrote:
> > Agreed. The compensation part is not the one I worry about. What 
> > worries me the most is the angle-in-radians part.
> >
> > Let's say that from the time sink plot I observe that I have pi/2 
> > radians phase offset between channel 1 and channel 2. How do I know 
> > it's pi/2 radians and not -3pi/2?
> >
> > To be precise, I am also dumping the 2 channels complex data to 2
> > file sinks, which I then import on matlab for easier and faster 
> > experimentation.
> >
> > In other words, how can I precisely calculate the 4 discrete phase 
> > shifts I am observing in my application?
> >
> > Should I use cross correlation?
> >
> > Should I use Hilbert transform?
> >
> > I don't have a very noisy application, in case that plays a very 
> > crucial role.
> >
> > Thanks.
> If you consult the "uhd_fft" app within the gnuradio/gr-uhd source
> tree, you'll see that it has a mode where it will display the
>    relative phases of two channels when you have a two-channel input.
> It also has a bunch of different synchronization options,
>    so you can glean a lot of technique info from that.
> 
> Also, there's a fair amount of "stuff" out there on phase
> measurements with Gnu Radio--this is just the first one that popped up
>    in a Google search:
> 
> https://www.egr.msu.edu/classes/ece480/capstone/spring14/group02/docs/Application%20Note%20-%20Phase%20George%20Godby%20Team%202.pdf
> 
> 
> >
> > On Jun 13, 2021 22:06, Marcus D Leech <patchvonbraun@gmail.com>
> > wrote:
> >
> >     Phase *correction* is easy once you know what that correction
> >     should be. Just a complex multiply-const by
> >
> >     Complex(math.cos(angle-in-radians),math.sin(angle-in-radians))
> >
> >     Or the equivalent in complex exponential notation.
> >
> >
> >
> >     Sent from my iPhone
> >
> >         On Jun 13, 2021, at 1:52 PM, Skyvalakis Konstantinos
> >         <kskyvalakis@isc.tuc.gr> wrote:
> >
> >         
> >         Understandable. However, if you eventually hear anything
> > about it please let me know.
> >
> >         Can I contact you again tomorrow for some questions I have
> >         about the phase correction block I need to make? (I mean
> >         through discuss-gnuradio)
> >
> >         Thank you very much.
> >
> >         On Jun 13, 2021 19:37, Marcus D Leech
> >         <patchvonbraun@gmail.com> wrote:
> >         I haven’t heard fro R&D on it. Part of the problem is the
> > N200 was designed over a decade ago, and the original engineering
> >         team have long since departed…
> >
> >         Sent from my iPhone
> >
> >             On Jun 13, 2021, at 6:25 AM, Skyvalakis Konstantinos
> >             <kskyvalakis@isc.tuc.gr> wrote:
> >
> >             
> >             Hi Marcus and sorry for bothering you again, but I'd
> > like to ask if you have any news regarding my problem.
> >
> >             Thank you.
> >
> >             On Jun 11, 2021 16:15, Skyvalakis Konstantinos
> >             <kskyvalakis@isc.tuc.gr> wrote:
> >
> >             ​Yes it kind of helps, because if I manage to apply a
> >             phase shifting/correction technique, then all 4
> > scenarios could produce sensible data.
> >
> >
> >             I am trying to build a phase correction block now, that
> >             will compare the 2 received signals, to see which phase
> >             shift out of the possible 4, minimizes the absolute
> > phase difference between the 2 channels.
> >
> >
> >             After I figure out which phase offset is the correct
> > one, out of all 4, then I can run the rest of my
> >             DoA application. It is a bit of a pain in the a**, but
> > it will have to do in case it never gets fixed.
> >
> >
> >             Let me know if you have any news from Ettus.
> >
> >
> >             Thanks again.
> >
> >
> >             
> > ------------------------------------------------------------------------
> >             *From:* Marcus D Leech <patchvonbraun@gmail.com>
> >             *Sent:* Friday, June 11, 2021 4:05 PM
> >             *To:* Skyvalakis Konstantinos
> >             *Subject:* Re: Phase Synchronize 2 USRP N200 w/ SBX
> > cards Also does knowing that there are only 4 possible phase
> >             relationships help in your application at all? Like only
> >             one of the 4 possible phase assumptions can produce
> >             “sensible” data? Just a thought.
> >
> >             Sent from my iPhone
> >
> >                 On Jun 11, 2021, at 9:03 AM, Marcus D Leech
> >                 <patchvonbraun@gmail.com> wrote:
> >
> >                 I am an Ettus support contractor. I have a direct
> >                 channel to R&D so I will relay any insights I
> > receive here since any such feedback would be generally useful.
> >
> >                 Sent from my iPhone
> >
> >                     On Jun 11, 2021, at 8:15 AM, Skyvalakis
> >                     Konstantinos <kskyvalakis@isc.tuc.gr> wrote:
> >
> >                     
> >
> >                     ​I also tried contacting Ettus Support but they
> >                     are not replying to my e-mails.
> >
> >
> >                     I will try to implement the phase calibration
> > for now.
> >
> >
> >                     Thank you for your effort.
> >
> >                     
> > ------------------------------------------------------------------------
> >                     *From:* Marcus D Leech <patchvonbraun@gmail.com>
> >                     *Sent:* Friday, June 11, 2021 2:23 PM
> >                     *To:* Skyvalakis Konstantinos
> >                     *Cc:* Discuss-gnuradio@gnu.org
> >                     *Subject:* Re: Phase Synchronize 2 USRP N200 w/
> >                     SBX cards
> >                     I have a query in to Ettus R&D about possible
> > causes.
> >
> >                     But you might need, for now to do an initial
> > phase Calibration when you start up.
> >
> >                     Sent from my iPhone
> >
> >                         On Jun 11, 2021, at 3:39 AM, Skyvalakis
> >                         Konstantinos <kskyvalakis@isc.tuc.gr> wrote:
> >
> >                         
> >
> >                         I repeated the experiments once again today
> >                         and I got once again the same results. I
> > still keep on randomly getting these 4 cases I
> >                         attached on a previous message.
> >
> >
> >                         I observed that cases 1.png and 3.png have a
> >                         phase difference of  +/- 180 degrees
> >
> >                         and cases 2.png and 4.png also have a phase
> >                         difference of  +/- 180 degrees.
> >
> >
> >                         Do you reckon you could help me any further
> >                         with my problem? I really need to achieve
> > this synchronization for my thesis.
> >
> >
> >                         Thank you very much.
> >
> >                         
> > ------------------------------------------------------------------------
> >                         *From:* Discuss-gnuradio
> >                         
> > <discuss-gnuradio-bounces+kskyvalakis=isc.tuc.gr@gnu.org>
> >                         on behalf of Skyvalakis Konstantinos
> >                         <kskyvalakis@isc.tuc.gr>
> >                         *Sent:* Friday, June 11, 2021 12:39 AM
> >                         *To:* Marcus D. Leech
> >                         *Cc:* Discuss-gnuradio@gnu.org
> >                         *Subject:* Re: Phase Synchronize 2 USRP N200
> >                         w/ SBX cards
> >                         Exactly my thoughts as well, I mean
> > regarding the phase ambiguity of the WBX daughterboards,
> >                         from what I've read on the Ettus website.
> >
> >                         I am 100% sure the daughterboards are SBX.
> >
> >
> >                         On Jun 11, 2021 00:26, "Marcus D. Leech"
> >                         <patchvonbraun@gmail.com> wrote:
> >
> >                             On 06/10/2021 01:00 PM, Skyvalakis
> >                             Konstantinos wrote:
> >
> >                                 In addition to my last message
> >                                 regarding the INTEGER_N tuning​, ​I
> >                                 repeated the experiment multiple
> >                                 times. I even restarted the USRPs
> > and the signal generator multiple times.
> >
> >
> >                                 What I observed was, that there were
> >                                 only 4 different recurring phase
> >                                 offsets (Blue = RX2 of SBX1, Red =
> > RX2 of SBX2).
> >
> >
> >                                 I am attaching the 4 screenshots I
> >                                 captured.
> >
> >
> >                                 Thank you so much for your help. I
> >                                 feel like we're getting there.
> >
> >
> >                                 
> > ------------------------------------------------------------------------
> >
> >                             You're sure that you have SBX cards and
> >                             not WBX cards?  Sorry for the
> >                             apparently-stupid question, but the WBX
> >                             uses almost the
> >                               same hardware as the SBX, but uses a
> >                             2XLO mixer that has fixed phase
> >                             ambiguity--which would look like what
> > you have here.
> >
> >
> >
> >
> 



-- 
JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000
Besancon, France