[time-nuts] OXCO insulation

[Richard (Rick) Karlquist]

On 2/25/2015 4:32 AM, Attila Kinali wrote:
> On Wed, 25 Feb 2015 10:16:58 +0100
> Attila Kinali <attila at kinali.ch> wrote:
>
>> Actually, you should put the temp sensor close to the heater, not the crystal.
>> The delay between the actuator (heater) and the feedback (temperature sensor)
>> defines the dead time. The presence of a crystal somewhere in the system is
>> of no importance to the oven controller, it's just additional thermal weight.
>
> Bob Camp just reminded me of Rick Karlquist's papers on this topic:
>
> "A low Profile High-Performance Crystal Oscillator for Timekeeping
> Applications", by Karlquist, Cutler, Ingman, Johnson, Parisek, 1997
> http://www.karlquist.com/osc.pdf
>
> "The Theory of Zero-Gradient Crystal Ovens",
> by Karlquist, Cutler, Ingman, Johnson, Parisek, 1997
> http://www.karlquist.com/oven.pdf
>
> More can be found on his homepage: http://www.karlquist.com/
>

A quick summary of the insulation issue in OCXO's:

Virtually any kind of foam, or just still air has similar
insulating properties in an OCXO.  Exotic kinds of insulation
like aerogel, or vacuum, are impractical because you have to
get rid of the heat generated by the electronics.  The E1938A
oscillator described in the above references achieved a thermal
gain in excess of 1,000,000 (in a single stage oven) in prototype 
testing, and routinely achieved many 100's or thousands on a production 
basis and had only 1/4 inch of foam that was nothing special thermally.
We also were able to get high thermal gain with a still air
experiment.

At HP, the main issues with foam were outgassing and mechanical
fatigue, like a mattress getting flat and the foam had to withstand
the oven temperature continuously.  Another issue was what
happens if the oven runs away and the foam burns and produces
toxic gases.  Generally, we are talking manufacturability issues.
The one thing the choice of foam had little to do with was performance.

Rick Karlquist N6RK