[time-nuts] Practical considerations making a lab standard with an LTE lite

Charles Steinmetz csteinmetz at yandex.com
Sun Nov 23 09:46:22 EST 2014

Poul-Henning wrote:

>Charles' design works great from the outside, but doesn't do anything
>with respect to the thermal energy expended by the encapsulated
>device themselves, which will cause convection in the inner box.

I have been using the technique for 30+ years, including with many 
OCXOs (which, obviously, generate significant heat) and have never 
observed any problems of that nature at the 1e-13 level.  I did 
consider the possibility when I first started doing it, and tested 
two potential fixes:  (i) putting a fan inside the box to homogenize 
the internal temperature, and (ii) filling the air space inside the 
box with irregular solid shapes to break up the convection 
pattern.  I tested both methods extensively with instrumented 
sources, in many variations (fan speeds and orientations, mass and 
porosity of passive internal shapes), and did not find any difference 
at the 1e-13 level.  I have occasionally used an internal fan just on 
theoretical grounds, but I have never measured any practical difference.

Thinking about it, this does not seem too surprising -- one would 
expect any convection to settle into a stable pattern and thus not to 
cause any temperature changes over time (once it is warm and 
settled).  Whether this explains my results or some other effect 
predominates (for example, convection may move enough air in the 
limited space to achieve substantial isothermy), I have confirmed to 
my satisfaction that it is simply not a factor in practice at the 
levels we are concerned with.

If you test the "cast aluminum box" method and find that your results 
do not accord with mine, please publish them and we can discuss what 
might account for the observed differences and how the method could 
be improved.  Until then, you are just posting speculative musings on 
the subject based on no data, which does not seem helpful.

And good luck fitting a cubic foot box with a surround of bricks into 
a 3U rack cabinet, or any other relocatable (much less, 
semi-portable) enclosure.

Best regards,


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