[time-nuts] Thermistor on OCXO

[Bill Dailey]

I am using a fury board with on off board oscillator.  So a thermistor is representing ocxo current.  Right now without tempco I have 1.7 ns sd in ti.  If I can eliminate "bumps" when the temperature changes slightly I can back the efc gain down below my current setpoint of 0.5.  Right now if I go any lower on efc gain the thermal changes perturb the system a bit too much.  When the temp is constant I can bring the efc gain way down...and therefore the sd to close to 1 ns.  What I am trying to achieve is a much more stable and narrow time interval.  Current performance is pretty close to as good as it gets for a gpsdo but I think I can do a little better.

Sent from my iPad

On Oct 17, 2012, at 5:55 PM, Azelio Boriani <azelio.boriani at screen.it> wrote:

> Yes, usually increasing temperature increases frequency, so if you see an
> oven current increase then slightly take down the EFC (and the other way
> around if the current decreases).
> 
> On Thu, Oct 18, 2012 at 12:47 AM, Rick Karlquist <richard at karlquist.com>wrote:
> 
>> 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
>>> is
>>> Datum 1111C... was thinking of using thermal epoxy to hold it down and
>>> then
>>> putting some insulating material (suggestions) on top of it.
>>> 
>>> 
>>> 
>>> --
>>> Doc
>>> 
>>> Bill Dailey
>>> KXØO
>> 
>> 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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