[time-nuts] First success with very simple, very low cost GPSDO
csteinmetz at yandex.com
Thu Apr 10 15:31:52 EDT 2014
>I should have said warm start, not cold. I was referring to the
>code, not the oscillator. So tell me, the OCXO is warm, there's no
>previous EFC information to draw upon, and the oscillator is
>off-frequency by more than can be measured with, let's say eight
>timer bits. What do those early measurements tell me, and which
>direction from midway should the EFC be adjusted?
That's why I suggested a timer. Certainly, the delay chosen for a
cold start would be excessive for a warm start, but I'm assuming that
the GPSDOs we're discussing are not used in life-and-death
circumstances where every second of unavailability is
critical. Whenever you power up -- warm or cold -- you wait
(probably ~ 5 minutes) for availability.
Using the PPS to discipline the oscillator during warmup may seem
like a good idea. However: (i) it will not be disciplined to useful
time-nuts standards both because it is drifting and because the
frequency is being set by the noisy, jittery GPS PPS signal. But
much worse, (ii) if the loop is fast enough to track the oscillator
as it warms up, it is almost certainly too fast to give best
performance at low to medium tau when the oscillator is warm, because
the noisy, jittery PPS will be contributing to stability at tau where
the nice, quiet OCXO should be in charge.
Precision takes time. Time nuts can't afford to be impatient.
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