[USRP-users] 2 N200 MIMO system phase offset varies with frequency, have used timed_command with tune and also integer-N Tuning per Marcus M post of Feb 17, 2016

John Shields john.shields at xtra.co.nz
Wed Oct 25 16:43:13 EDT 2017


Thanks Marcus,
                         I take your advice re: ‘calibration’ and DRAO. 

                         I had hoped, however, that I would not be dealing with > 100 degree offset in an ideal environment i.e. same signal through a good quality splitter positioned right at the input to the SBXs. While it was immediately obvious when you mentioned that the MIMO implementation makes no correction for the delay (not even the roughest based on the length and velocity factor which would not cover eveerything), it does mean (to me at least) that I am dealing with a larger offset than I thought from the Ettus documentation with all the talk about synching SBX LO etc. and, while mentioning there are component factors which mean that the offset was not zero, not highlighting there is a deliberate frequency sensitive offset built in to the design of the ‘'MIMO cable”.

                        Will ponder my next move. 

                                  Kind Regards,

                                             John

From: Marcus D. Leech 
Sent: Thursday, October 26, 2017 3:56 AM
To: John Shields 
Cc: usrp-users 
Subject: Re: [USRP-users] 2 N200 MIMO system phase offset varies with frequency, have used timed_command with tune and also integer-N Tuning per Marcus M post of Feb 17, 2016

On 10/25/2017 03:16 AM, John Shields wrote:

  Thanks very much Marcus for the thorough explanations. I looked at the phase change with frequency to see if there was a fixed delay and there didn’t appear to be but, effectively, the MIMO cable induces a frequency dependent uncorrected phase offset  which is eminently understandable but would appear to make a mockery of ‘MIMO’ claims. I realised that there will always be a phase offset but was disappointed by the magnitude as measured by the complex conjugate of both signals, a complex_to_arg block and decimating the result by 1K and plotting on Qt GUI Time Sink.
In commercial MIMO applications, the implementation corrects for phase-offset error, because it is (reasonably) expected that there will always be
  some amount of phase offset.  It's inevitable for there to be *some*.   For example, the DRAO synthesis array uses hardware to measure the
  phase length of each of their cables, and corrects for thermal-expansion effects in real-time.  Since phase-offset error (and drift) extends outwards
  away from the USRP envelope, it's not realistic to expect that all such effects are accounted for in the hardware (at least, not without a much
  higher price-tag).



  It would appear that if I wanted to try to get as close to zero phase offset (to correct for non-zero MIMO cable length at least), then I need an Octoclock-G but I don’t have the nearly  $NZD 3000.00 it costs so I wonder if there is a ‘cheap’ way to convert my existing GPSDO board into an Octoclock-G? I only need to be able to buffer the signal for 2 USRPs.
You could just try splitting the signal two ways.   Myself, I buffer such signals with 74HC04 inverters, but I'm handy with a soldering iron.  There are
  cheap GPSDOs out there now, so it's just a matter of buffering, and for only 2 units, you might be able to get away with just splitting them.




  Otherwise, I guess I could ‘calibrate’ the offset at various frequencies and then, at run time, apply a phase correction to one leg based on the fc? Seems a little inelegant.
That is *PRECISELY* what most MIMO applications do, and folks doing beam-forming, etc.  There will ALWAYS be some amount of phase offset--
  some of it fixed, some of it variable.   It is a *systemic* imperfection, which means that it has to be corrected for that account for all the systemic
  contributions, including hardware entirely outside of the USRP.




              Kind Regards,

                         John

  From: Marcus D. Leech 
  Sent: Wednesday, October 25, 2017 1:20 PM
  To: John Shields 
  Cc: usrp-users 
  Subject: Re: [USRP-users] 2 N200 MIMO system phase offset varies with frequency, have used timed_command with tune and also integer-N Tuning per Marcus M post of Feb 17, 2016

  On 10/24/2017 07:45 PM, John Shields wrote:

    Thanks Marcus,
                            So it appears that the synching of the SBX LOs doesn’t work; or perhaps I should say, it doesn’t work during my measurement period? The integer-N tuning doesn’t work either. 

                            I can say that, with some level of precision, the phase is fairly constant with center-frequency but if, for example, I had a 5 MHz spectrum how could I ‘correct for that’? I be;ieve that there is the whole Hilbert transform issue when you wish to translate the phase/frequency of a band of signals to a different one –is that what I should use?

                            From my point of view, there is quite a misinterpretation of what ‘synchronistation’ means; in particular for SBXs and their LOs which, as advertised, are supposed to be capable of such operation with a few simple Python commands!.

                            Realising that you would/should not express some shortcoming in the SBX,N200,MIMO in an Ettus product , if there is, I would dearly like to know from someone from Ettus!!!! Purely from an outside point of view, I thought that the “ we’ll transfer the Time Of Day contents to the Mate over MIMO cable ” doesn’t actually mean that they are in ‘real time’ synch, from my old DMS-100 days bit was willing to go along with the theory. Seriously, I have no issue with that but just want to know how to get 2 N200r4 streams with OB GPSDO & MIMO cable ‘synchronised’ 

                           I would love (but be embarrassed) to be told, that as a dummy, I made this mistake but in over a month of work I have not been able to establish that.

                          Kind Regards,

                                       John

  Set up a test transmitter in the far-field of your two antenna.

  With everything synchronized the way you think it should be, plot the low-pass-filtered (and decimate to taste) result of a conjugate multiply of
    the two sides.   This should produce a straight line, with small amounts of noise.   If it just produces random walks all over the place, the two
    oscillators aren't locked to the same reference.

  My point about component tolerances is that they'll have some group-delay that isn't perfectly matched on both sides, even if things like the
    LO are running in-phase, the analog pathways won't necessarily have precisely the same group delay on the two sides.  Just like two random
    pieces of coax that are cut to the same length won't, necessarily, have precisely the same phase length.   This effect gets worse with frequency.

  Further, in radio astronomy applications, the coherence bandwidth is, technically speaking, infinitely small, due to the emission mechanisms.
    But in *practice* a significant fractional bandwidth is possible without having to channelize the input bandwidth.

  The *other* issue, that seems to be causing consternation, is the ability to predict what the phase-offset between the two sides will be upon restart
    of the flow-graph in the presence of the various bits of hocus-pocus (timed commands, etc) to try for consistent phase offsets every time you
    start streaming.  It sounds like you have that, but the offset changes depending on tuned frequency.   I'd expect that.  Both due to analog-component
    group-delay variability, and because the MIMO cable is not of zero length.  I don't believe that there is *ANY* length compensation, so one N2XX will
    receive the reference clock at a "closer" phase distance than the other one, because the MIMO cable is of finite length.  That phase-length difference will
    have more effect at higher frequencies, because a PLL is a reference multiplier (which is why having exquisitely-low phase-noise on the reference is
    important, because that noise will get worse as the multiplier ratio of the PLL increases).





    From: mleech at ripnet.com 
    Sent: Wednesday, October 25, 2017 7:45 AM
    To: John Shields 
    Cc: usrp-users 
    Subject: Re: [USRP-users] 2 N200 MIMO system phase offset varies with frequency, have used timed_command with tune and also integer-N Tuning per Marcus M post of Feb 17, 2016

    I would expect component tolerance issues on the two sides to scale with frequency.  That may be what you're seeing?








    On 2017-10-24 14:28, John Shields via USRP-users wrote:

      Hi,
          Still struggling with the configuration – 2x N200 r4, master O/B GPSDO, slave MIMO cable. Have put in python code to use timed commands and that produced a constant phase offset even over rerun of FG or power cycling on N200 which was great news.

          However, the relative offset changes with frequency. The splitter is a Mini-circuits ZRFSC-123S+ which is spec-ed to has a typical of phase unbalance of 1/2 a degree over the frequency ranges used. The results are independent of source NWT 4000-1 or an SBX using uhd_siggen. When I have checked the ref_locked flags etc. they are good. the gpsdo is 'locked' as is MIMO.

          In addition to using the timed_commands to synch the SBX LOs, I also implement the integer-N-tuning and no improvement.

          The results are roughly    Freq (MHz)    Phase offset (deg)
                                                  450                    -7
                                                  1450                -30
                                                  1950                -65
                                                  2450                -100

          When I switch the cables between the 2 N200, the phase offset doesn't change sign so I presume it is not cabling? What on earth, else, could it be?

                Kind Regards,

                         John

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