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
>
> [1]
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>
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