[time-nuts] Thermistor on OCXO
richard at karlquist.com
Wed Oct 17 18:47:48 EDT 2012
Bill Dailey wrote:
> If I wanted to try to compensate for temperature variation of my OCXO...
> where would I attach a thermistor? What would be a good method of
> attachment to best respresent the OCXO temp without opening the OCXO and
> without an undue influence of ambient temperature? The OCXO in question
> Datum 1111C... was thinking of using thermal epoxy to hold it down and
> putting some insulating material (suggestions) on top of it.
> Bill Dailey
Probably easier to monitor oven current and use it to tweak the
set point of the oven or tweak the EFC line of the oscillator.
As I explained in my 1997 FCS paper on zero gradient ovens,
the gradient is the main thing limiting thermal gain. If you
have a serious gradient problem, then locating the thermistor
far away from the crystal (probably much farther away than the
internal one) isn't likely to be productive.
The other issue to be aware of is that at some point, the
tempco of the electronics comes in play, as explained in my
1997 FCS paper on bridge stabilized oscillators. Increasing
the thermal gain of the 10811 beyond 1,000 doesn't improve
the tempco because of oscillator circuit pulling. You could
possibly improve the tempco by using oven current to tweak
the set point. This might cancel out circuit pulling drift,
regardless of what was going on with the crystal. I've never
actually tried this, but it would be good research project
well within the ability of any time nut.
Rick Karlquist N6RK
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