[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

mleech at ripnet.com mleech at ripnet.com
Tue Oct 24 14:45:15 EDT 2017


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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