[time-nuts] Morion MV89A position

Charles Steinmetz csteinmetz at yandex.com
Wed Jan 29 15:31:43 UTC 2014


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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