On 06/05/2020 04:35 PM, Sidd Subramanyam wrote:
By clock bias I am referring to the difference between true GPS time
and the USRP time.
For example, in my application I am running 2 Separate B210 USRPs each
side by side with a split signal coming from a single S band antenna.
However, they are both disciplined using separate GPSDOs with separate
antennas to simulate being 2 independent devices. They are being
synchronized as described in method 2 in this article
(https://files.ettus.com/manual/page_sync.html) and then the data
collection is being commanded to start at the same GPS time for both
devices.
My goal is to try to create as phase coherent of an operation as
possible between the 2 USRPs.
When I cross correlate the 2 split S band signals to calculate
difference in phase between the 2 signals, there is a significant
drift over time. This phase difference converted is about 300
nanoseconds over the course of 1 second (300 PPB).Because of this, I
wished to see if I could use the calculated clock bias that I
described above to compensate for this drift in phase.
However, now that you have mentioned that I should in fact be getting
1 PPB accuracy when synchronized to GPS time, I am questioning if I am
doing the time synchronization process wrong altogether Since I seem
to be getting drift around 300 PB after following the instructions in
the link above.
Sidd
It would be useful to see the parts of your source-code that set up your
devices--including clock synch, etc.
Further, I'll note that two randomly chosen GPSDO units, even connected
to the same antenna, will be producing a 1PPS pulse up to several
(possibly 10s) of nanoseconds different from one another--which is
the typical spec on the 1PPS pulse.
In *addition* to this, two (or more) GPSDOs will not produce a 10MHz
output that precisely track each other, even when connected to the
same antenna. Their long-term behavior is good, but short-term, they
can disagree with one another, in my experience, so building a
perfectly-coherent system from two separate GPSDO units doesn't work
nearly as well as you might hope, particularly not a lower-cost
unit such as is used in the B2xx (where "lower cost" is kind of a
relative term--the high-end GPSDOs that use a local Rb oscillator are
considerably pricey, but not easily packaged into something like the
USRP B210).
_______________________________________________
USRP-users mailing list
[email protected]
http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
