On 2022-02-03 15:17, John Hodgins wrote:
Hi there,
We have a simple set up where we collect iq snippets from 3 usrp's at
the same rx-times via timed commands. We introduce delays between the
usrp's via a reference delay box and try to recover the introduced
delays via cross-correlation.
Things work great when all the rx's belong to the same USRP family.
For instance we have been able to recover the correct delay for all
B210 (with GPSDO/TCXO) or all E310 (with GPS antenna) scenarios.
However, when we mix different USRP's, the delays computed via
cross-correlation no longer make sense.
For instance, when we use 1 B210 and 2 E310's the delays between B210
and E310's just oscillate wildly but the relative difference between
E310 remains relatively stable. We tested 2 B210's with a single X310
(with GPSDO/OCXO) as well and we see the same problem as well. The
interesting thing is though visualizing the spectrum of collected
snippets from different USRPs show pretty much the spectrum of the
known applied signal.
Is there some fundamental discrepancy between different USRP families?
Or maybe UHD somehow interprets samples differently for different
USRP's (flipping i and.q, different rx-time, pps implementations,
some precision issues maybe)?
In any case I would be grateful if anyone can shed some light onto
this behavior.
Best,
John
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The group delays between USRP families will be different due to
different FPGA implementations, the exact "placement" of time-of-day
registers, etc.
It would be very difficult to guarantee phase-coherence across different
USRP device types, because the implementations are different--not just
the FPGA bits and
pieces (DUCs, DDCs, etc), but also the analog stages will be different.
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