[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 at xtra.co.nz
Tue Oct 24 19:45:38 EDT 2017
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.
From: mleech at ripnet.com
Sent: Wednesday, October 25, 2017 7:45 AM
To: John Shields
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:
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)
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?
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