Bryan wrote:
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 second.)
Best regards, Charles
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