Lux, Jim (337C) wrote:
-----Original Message-----
From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On
Behalf Of Ed Palmer
Sent: Thursday, August 27, 2009 12:06 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Couple of questions for Racal 1992 owners
Thanks for the info Antonio. Racal didn't spec the warmup
characteristics of the counter as a whole and it seemed a little odd
that the warmup drift for the counter was as large as 10 days of
oscillator drift.
Ed
Intuitively, this wouldn't surprise me. You set the OCXO up so that the
temperature is somewhat higher than the highest expected ambient (so, say, it's
at 60C) and pick a crystal that has a flat freq/temp characteristic near there.
So, when you turn on, your crystal is at, say, 25C, where the slope of the
freq/temp curve is pretty steep. I'll bet someone has some curves out there
that could give a quick order of magnitude estimate...
I have a poor quality chart here that shows an AT cut with about 40 ppm change
for a 50 degree C change, and an SC changes about 5 ppm over the same span.
Since a decent oscillator has aging of small ppm/year, that's orders of
magnitude bigger for the temperature effect.
Is this a SC or an AT cut?
I agree with what you've said, but on the 1991/1992 counter, there's a
standby function which keeps the oscillator hot, to avoid the issue
you've stated above. So the 10 days worth of drift when you turn the
counter on is just due to the counter warming up, not the oscillator.
The oscillators that I have (opt. 4A and 4E) are both AT crystals. This
isn't stated anywhere that I can find, but the frequency at startup is
in the range of 20ppm high which is consistent with an AT cut crystal.
Ed
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