[time-nuts] OCXO startup transient
Magnus Danielson
magnus at rubidium.dyndns.org
Sun Jun 28 11:19:30 UTC 2009
Hal Murray wrote:
> A couple of graphs in case anybody is looking for examples...
>
> This is a 3 MHz OCXO from Telco surplus.
>
> The can says EROS-800-M45, 94-12. I haven't found a data sheet but it's
> supposed to be equivalent to Isotemp 127-10.
>
> It's got a mechanical adjustment, no electrical adjustment. (I haven't
> touched the adjustment.)
>
> The can is 2.5 x 2.75 x 1.75 inches. I'd call it medium quality. Nothing
> fancy, but probably better than most of the small ovens. When I was playing
> a while ago, I could see changes when I grabbed the can with my fingers.
>
> It was powered off for at least 6 months before I started this run.
>
>
> I'm collecting samples every 5 minutes. I discarded the first 3 samples
> after power on. The first was way low. (It's so far off that the rest of
> any graph gets compressed into a flat line.) The next two overshot.
>
> When I run out of other things to do, I'll make another run with samples
> closer together for the first few hours. But first I want to watch the drift
> some more.
>
> So far, the drift has generally been in one direction. That's when averaged
> over several days. There is enough noise on the hour scale that I wouldn't
> want to try to predict anything to get better holdover.
Your holdover properties should be evaluated after running for say 30
days. A frequency compensation get you a first degree correction which
together with TDEV measures give you an indication of aggregated
holdover properties.
> To my eye, there is a bend in the drift curve at 4 days. I can't think of
> any environmental change that happened then. I might have bumped it.
Considering the wobble on day 8, I'd say the day 4 shift is within what
environmental shifts do to you, where as on day 4 the curvature is still
steep so the added effects fools the eye.
Cheers,
Magnus
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