On 09/06/2017 06:09 PM, Cho, Daniel J (332C) wrote:
So they are seeing exactly the same PPS source. I got a waveform
generator connected to a power splitter which is then fed into both
“PPS in” inputs. Since they are seeing the same PPS from the same
source, shouldn’t they be referencing off of the external source? Why
are they choosing an internal clocking source when I am feeding both
of them the same external PPS and choosing the option of using the
external clocking source on GnuRadio? What am I doing wrong and how
can I fix it?
Thanks,
Daniel Cho
*
I think that perhaps you are confused about what the 1PPS input does,
and what the difference between the "reference clock" and
"time source" are.
1PPS is simply used as a triggering event, to allow two (or more) USRP
devices to agree on the time-of-day (for timestamping purposes)
to within a very small margin of error (a few nanoseconds).
The *reference clock* input is used to drive the clocking system for the
entire board, including ADCs, reference clocks for RF synthesizers,
etc, etc.
In order for two or more streams to be time-synchronous *AND*
phase-coherent, you need both a 1PPS input to use as a trigger to
reset the time-of-day clock on the two-or-more USRPs, and the
reference clocks must all come from the same source. In your case,
you're only supplying the 1PPS. Even with each board having an
onboard GPSDO (that is actually "watching the sky"), they won't
agree completely on reference clock phase, and they will drift in
phase relative to one another a little bit, and will have relative
phase-noise--it will be *better*, but since the phase noise of the
oscillators in two GPSDOs aren't precisely the same, even when
being "steered" by the GPS signals, it won't be nearly as good as a
shared reference clock and 1PPS.
*
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