Poul-Henning Kamp wrote:
Agreed. The cast aluminum box I mentioned in my previous post is a
good way to add thermal capacitance without adding much thermal resistance.
That's exactly what you do not want.
Your aluminium box will very efficiently transport temperature
changes from its full surface to your OCXO.
I don't believe you understand how it works. The air space in the
enclosure isolates the oscillator from the cast box. The box is
sufficiently massive that its temperature cannot change nearly as
fast as ambient, and it can be adjusted in this respect by adding
thermal mass as desired. I have used this system with great success
many times in the past, and it will reliably reduce the slope of fast
ambient temperature changes by 20:1 or more at the oscillator, which
is plenty to allow the oven controller to keep up. As I mentioned,
those who feel the need can put the aluminum box in another enclosure
and regulate its temperature with a fan controller. I have rarely
seen any improvement in an oscillator's thermal stability by doing
this, but it can easily hold the temperature of the cast box (and the
air inside it) constant to well within 0.1C.
A plastic box with lid and filled with dry sand will do much better
than your alubox.
I experimented with dry sand and found that its thermal resistance
increased faster than its thermal capacitance. By the time there was
sufficient capacitance there was way too much thermal resistance.
Best regards,
Charles
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