[time-nuts] Linear voltage regulator hints...

Charles Steinmetz csteinmetz at yandex.com
Sun Dec 14 01:07:58 EST 2014

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,


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