[time-nuts] Frequency comparison
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Mon Feb 8 23:50:57 UTC 2010
>Yes, I noticed these things take time ;-)
>This is what I logged after everthing was powered on for over 24 hours
>And this is with the LPRO is cold started. About 5 minutes after lock.
>The GPS was already running for about an hour I have to guess.
>It is less flat at the last two hours than what I was hoping for.
There is a reason that precision oscillators are often specified for
stability "after running continuously for a period of [x]," where "x"
is generally from one week to 90 days, sometimes even longer. They
are not really made to be turned on and off, even if you let them
settle for a matter of hours. Even with a GPSDO, the performance for
small tau isn't really at its best until the unit has been on for
weeks to months. What range of tau is "small" depends on how you
tune your disciplining dynamics. I have found that the Tbolts in my
limited very sample (2 units) work best with TC = 500 to 2000 seconds
(bear in mind that I don't care about the PPS output, just the 10 MHz
frequency), so for me "small" tau are those less than about
1000. Note that the improvement in small-tau stability probably
won't appear in an ADEV plot that uses the received GPS time for
comparison, because the oscillator is more stable than the GPS
receiver at small tau -- so you're essentially measuring the GPS, not
It is best to leave your stable quartz and Rubidium oscillators
running continuously, preferably on a UPS in case of power failure,
for best results.
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