[time-nuts] Morion MV89A position

Charles Steinmetz csteinmetz at yandex.com
Wed Jan 29 13:21:46 UTC 2014


Poul-Henning Kamp wrote:

> >Warren pointed out that the MV89 has a double oven and said that this
> >makes added thermal capacitance unnecessary.
>    *   *   *
>Double oven OCXOs, in particular "high-end" models, are usually
>much better thermally insulated and therefore draw a lot less heating
>current.
>
>That is not a problem when they are exposed to sudden cooling, they
>can regulate heating up as fast as they need.
>
>But when they are exposed to sudden heating, they cannot regulate
>the heating current negative.
>    *   *   *
>I have seen this assymetry with a number of double oven OCXOs.
>
>The best way to mitigate it, is to make sure the temperature does
>not rise rapidly.
>
>Unfortunately, that is almost the most common failure case:
>A/C or local fans failing.
>    *   *   *
>What you want is to wrap your OCXO in a thermal impedance.
>
>The best result I have managed so far, was by wrapping the OCXO in
>domesticated geology, (bricks, concrete, cinderblocks etc), which
>has high-ish thermal capacity but only moderate thermal conductance.

Agreed.  The cast aluminum box I mentioned in my previous post is a 
good way to add thermal capacitance without adding much thermal 
resistance.  If the oscillator has a "thermal surface" (one face that 
is the primary path for cooling to ambient), you can mount that 
surface to a thick slab of aluminum that weighs a kg or more.  It is 
common for rubidium oscillators to have a thermal surface, but NOT 
for quartz oscillators, so the cast box is still the preferred 
solution for a quartz oscillator, IMO.  You can make the box as 
massive as you like -- just bolt it to a slab of aluminum with some 
thermal compound.  Make sure there is no direct conductive path to 
ambient (i.e., box plus added mass [if any] is cooled by convection 
and radiation only).

Best regards,

Charles






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