Use "unknown PPS" on both of them.  The MIMO cable shares both REFCLOCK
and 1PPS signals, so they will both be synchronized to the same time. 

On 2017-09-12 16:13, John Shields wrote:

> Thanks Derek,
> No, I hadn't been power cycling between the runs - good point, obviously, I 
> should have.
> 
> In terms of the 10 MHz and 1 pps references, in the configuration I was 
> testing, I don't believe so in that I had the MIMO cable disconnected. My 
> strategy was to have 2 USRPs with no MIMO - expecting little synchonisation. 
> Then I was going to add the devices into the same container and connect the 
> MIMO cable and expected things to improve and lastly, I was going to 
> hand-code the SBX phase synch code.
> 
> In terms of the version of UHD, the fg shows: <<< Welcome to GNU Radio 
> Companion 3.7.11.1 >>>
> 
> Thanks Marcus,
> I will implement your way of measuring the running phase offset and also 
> thanks for correcting my understanding of O/B GPS .
> 
> In terms of getting the devices in the container to be the best synch they 
> can be, I presume for the device which has the GPS, for the clock source and 
> time source, I would put O/B GPS for the device which has it and for the 
> other, I would put MIMO cable for both but in terms of the 'Sync' field, 
> where the options are PC Clock, Unknown PPS and Don't Sync, which option 
> should I select?
> 
> Thanks again for your help.
> 
> Kind Regards,
> 
> John
> 
> On 11/09/17 09:00, Derek Kozel wrote: 
> 
> Hi John,
> 
> Are you power cycling the USRPs between tests or just rerunning the GRC 
> flowgraph? Do you have shared 10 MHz and 1 PPS references? What version of 
> UHD is printed in the output? Thanks, Derek 
> 
> On Mon, Sep 11, 2017 at 1:50 AM, John Shields <sla1nte2...@gmail.com> wrote:
> 
> Thanks for the feedback but I am not sure that I understand it. What I was 
> hoping to do was step through the configurations with increasing levels of 
> synchronisation and expecting to see same.
> 
> Marcus' comment is correct and I have not, yet, put in the code which 
> synchronises SBXs.
> 
> I guess my basic point, from looking at previous post from others Marcus L 
> included, was that UHD would somehow improve the synchronisation between two 
> USRPs in the same container versus those two separately. 
> 
> When I executed the FG shown (separately) with the USRPs individually and 
> then within a UHD container the results in terms of phase variation was the 
> same. I had expected that, based on my understanding, the containerised USRPs 
> would have behaved better.
> 
> So, either my FG does not measure what I thought it should or there is little 
> UHD-related benefit to having USRPs individually or in the 'domain' as 
> MarcusL has mentioned previously. From my situation it doesn't hence the 
> first question in the post: 
> 
> Does my FG not measure what I claim to be wishing to measure?
> 
> Kind Regards,
> 
> John 
> 
> On 11/09/17 01:03, Marcus D. Leech wrote: 
> 
> On 09/10/2017 08:58 PM, Dan CaJacob wrote: 
> 
> I could be wrong, but I thought the SBX was one of the few daughter cards 
> that starts with s known phase offset? Only if you ask it to do so, and only 
> if it's sharing clock with its buddies...
> 
> On Sun, Sep 10, 2017, 2:49 PM Fulcrum Associates <sla1nte2...@gmail.com> 
> wrote: Dear All,
> 
> I have a couple of USRPs connected, through  a strong
> attenuator to a signal generator (NWT4001). While the units have a MIMO
> option, I don't have that cable. (Option A) When I run the GRC as
> attached, I see too good a result to the extent that the differential
> Phi seems to range over +/- 5 degrees.
> 
> What I had hoped to prove to myself that two N200 with SBX
> would have a varying offset without MIMO cable, then I would connect the
> MIMO cable and move the USRPs into a multi-unit and enable GPSD O/B on
> the unit which has the feature and MIMO for one without (Option B) and
> that the phase differential would improve noticeably and be a variable
> constant, but it didn't.
> 
> If it had, but there still was a fixed phase offset which
> varied each time it was setup (which is what I would expect under B)
> then I would hand-code the SBX stream initialisation code to remove the
> offset.
> 
> Does my FG not measure what I claim to be wishing to
> measure?
> 
> If it does measure it correctly, why do my expectations
> of options A and B leading to a different (though improved) situation
> not eventuate?
> 
> Kind Regards,
> 
> John
> 
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> -- 
> 
> Very Respectfully, 
> 
> Dan CaJacob 
> 
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