[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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