[time-nuts] Linear voltage regulator hints...
csteinmetz at yandex.com
Sun Dec 14 01:07:58 EST 2014
>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.
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