[time-nuts] Thunderbolt performance vs temperature sensor

Magnus Danielson magnus at rubidium.dyndns.org
Sun Mar 8 20:13:32 EDT 2009


Steve Rooke skrev:
> 2009/3/9 Magnus Danielson <magnus at rubidium.dyndns.org>:
> 
>> So, you can expect that the metal can of the OCXO reacts fairly quickly
>> and the temperature sensor is a thad slow. This also matches exercises
>> we have done by using a fan to do forced air convection tests on OCXO
>> and tempsensor. Putting a *plastic* hood over it significantly lowered
>> the risetime and the consequence is that the lower risetime allowed the
>> tempsensor inside the OCXO to sense the change and react to it. This
>> introduces a third time constant, the time-constant of the OCXO control.
>> The effect of using a hood or not was significant. The change of
>> frequency shape was distinct and for the measurement interval we looked
>> at the phase drift was about 1/3 for this simple addition.
> 
> So would boxing in an ocxo with something like polystyrene foam
> improve the tc of the unit? Thinking about this it is a stage towards
> having a double oven.

Yes, it is a step towards a double oven. Regardless of how many oven 
stages you have, letting the outermost oven be too unisolated will 
partly defeat it, which is somewhat my point here.

There are problems with too much isolation too. For an oven to work it 
needs dynamics in both directions, so it actually needs to loose some of 
the heat in order to handle the situation when the outside is quite hot.

The trick here is just to get the dynamics time constants down, but 
maintaining sufficient cooling load.

Cheers,
Magnus




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