[time-nuts] OCXO startup transient

Hal Murray hmurray at megapathdsl.net
Sat Jun 27 23:22:51 UTC 2009

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.

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.

For my purposes, it takes a few hours to a few days to settle down, depending 
upon how stable you want it.

Watch for scale changes.

First 3 hours:

First 3 days:

First 10 days:

Days 2-12

These are my opinions, not necessarily my employer's.  I hate spam.

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