[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
Tue Oct 24 14:28:19 EDT 2017


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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