[time-nuts] Expected 10 MHz offset from a GPSDO?
csteinmetz at yandex.com
Sat Aug 20 07:38:11 EDT 2016
> With a GPSDO as a general rule is there much variation of the 0.005 to 0.02 ppb range. Would there me much if only 2 satellites were visible but the unit was no in holdover vs say 6 satellites. I assume there is probably a lot of variables such as quality of GPSDO and OCXO, multi path interference, antenna location, etc, but worse case what would a user expect assuming the unit isn't in holdover.
There are lots of ADEV plots of GPSDOs on the web, if you search for
them. Tom has the attached plot posted on his web site -- it includes
one decent (but not stellar) GPSDO, the HP Z3801. That is about what
you can expect without heroic tuning efforts (on a GPSDO that supports
user fiddling, such as the Trimble Thunderbolt).
Since many counters gate for one second (or gate faster, then average),
the ADEV at tau=1 second is a reasonable estimate of the real-world
performance of a good counter driven by a GPSDO. Note that, since the
ADEV at 1 second depends largely on the OCXO, the heroic tuning efforts
mentioned above won't change the ADEV at tau=1 second hardly at all.
That is attributable strictly to the OCXO (unless the GPSDO is very
poorly designed). Building a GPSDO using the best OCXO available can
improve things. For example, look at the "BVA" plot -- at 1 second, its
ADEV is nearly a full decade better than the Z3801, better even than the
hydrogen masers. (While I referred to the 3801 above as "decent but not
stellar," that is based on its performance at tau from 100-100k seconds.
Its OCXO -- and, thus, its ADEV -- is better than most others at 1
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