[time-nuts] Counter OCXO behaviour

David davidwhess at gmail.com
Mon Jan 14 21:04:29 UTC 2013


On Mon, 14 Jan 2013 21:14:23 +0100, Fabio Eboli <FabioEb at quipo.it>
wrote:

>Hello,
>recently I tried to trim the Racal Dana
>1992 04E internal reference, using the
>GPS pps as a reference. I'd like to ask
>a pair of questions...
>
>- First is about the method.
>I'm using the counter TI to measure it's
>own OCXO. The GPS is starting the count,
>the internal reference 10MHz (on the rear
>there is a reference output connector)
>stops the count and I log the result
>as I've done before with other oscillators.
>Is this a correct procedure?

I just measured the GPS pulse output directly with the counter.  I got
the same calibration results measuring frequency or period.  With time
interval, I used the delay feature to set the minimum measurement
duration.

>- Second, is about the behaviour of the OCXO
>after trimming. The OCXO seem not that stable
>after the trimming, like if the crystal started
>to age faster than before trimming, and now is
>slowly stabilizing.
>It's like the retrace of the crystals I've read
>about, but the instrument was never powered down
>in the last month. And it's oven has been on for
>the last year. Will a crystal retrace also
>after retrimming?
>The frequency of the crystal is slowing, in the
>first hours after the trimming rapidly, and now
>more slowly. After few days the frequency seem
>to be slowing somewhere around 1x10^-10 per day.
>Unfortunately I havent logged the counter for
>enough time before the trimming, but it measured
>the same Rb with less than 3 digits of difference
>(3x10^-10) in last 6 monthes.

My 1992 has the TCXO and I noticed the least significant digit
drifting after trimming.  At least with the 1992 TCXO, I decided the
last digit was not worth worrying about which is pretty much the case
with all of my other counters although my 1992 is about the best of my
bunch.



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