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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