Dan wrote:
The test was simple. A couple of seconds of warm air from a hair dryer on low setting (the air coming out is 15C above ambient, so not very much heat at all) through a paper tube blowing on the GPS. That little change causes the phase to shift about 200nS in just a few seconds. That is, the GPS PPS phase compared to the OCXO phase shifts 200nS, and it happens over only a few seconds, literally almost instantly. It's an enormous change!
LOL. A couple of seconds of warm air at 15C above ambient is a HUGE temperature transient for any sensitive electronics, especially anything with an oscillator. I would venture a guess that the lion's share of the drift you see is the GPS time base shooting off-frequency, but there are probably other effects, too (voltage regulators, to name just one).
To me, a "little change" in this context might be blowing one warm breath toward the GPS unit from 18" away and seeing what happens over the next minute or two.
But the GPS temperature sensitivity shouldn't be a big factor in actual use. The GPS should be thermally isolated from anything that changes temperature rapidly, and enclosed such that external temperature changes are integrated over at least tens of minutes. Then, the inside of the enclosure will reach its own thermal equilibrium and any external changes will be slowed enough to be tracked out by the GPS discipline. My recommendation would be to put it in a cast aluminum box (search the archives for "cast aluminum box"), but there are others who think you need to build a two foot cube out of cinderblocks and fire brick against a wall in the deepest external corner of your basement.
OR, if my suspicion is correct that the temperature sensitivity is mostly the GPS time base, figure out a way to kludge the GPS to accept the disciplined OCXO as its time base.
Best regards, Charles _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
