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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