On 09/11/2017 04:50 AM, John Shields 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.
What the multi_usrp object will do is to *align timestamps* among all
the devices within the "container". That is distinct from arranging for
synchronization in time and phase. For that, you must have a shared
10MHz reference and 1PPS source, and you must ask the
UHD source to use them.
What I do for testing ongoing phase coherence is to conjugate multiply
the two halves, then low-pass filter and decimate. This allows me to
observe any phase-drift between the two sides over time. This
assumes that both sides are tuned to some common test signal.
In the case where two devices aren't sharing a reference clock, one can
expect the above to produce a noise-like output, since there will be random
mutual phase noise. In the case where the two sides have a slow
phase-drift with respect to one another, one can expect a slow
sinusoidal output.
In the case where both sides are in constant phase coherence, one can
expect an essentially-constant output.
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 <mailto: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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