[time-nuts] Morion MV89A position

[Charles Steinmetz]

Paul wrote:

>1) Is the fact that the mv89a is inside a Dewar causing any short 
>term or long term adverse effects?

Dewars are appropriate for OCXOs that are designed to work in 
Dewars.  They do not generally improve the temperature regulation of 
OCXOs that were not designed to work in Dewars, and can significantly 
compromise the temperature regulation of some OCXOs.

An oven works by heating the crystal (and often other circuitry) with 
a heating element (this is the "pull up"), which is balanced by heat 
loss to the ambient environment (this is the "pull down").  By 
putting the MV89 in a Dewar, you have reduced the "pull down" that 
balances the heater.  This means that the heater must stably deliver 
much less heat than it was designed to, and the heater control 
circuitry is operating far from its target design point.  It may have 
enough range to work this way, but it is not operating at the 
design-center "sweet spot" that the designers chose.

This applies to any OCXO, and any form of insulation you use.  What 
you want to achieve is a "pull-down" rate (rate of heat loss to 
ambient) similar to the rate anticipated by the thermal designers, 
but with some integration to slow down the change in cooling rate 
that occurs when the ambient temperature changes quickly.  This gives 
the heater control servo more time to adjust, thereby improving 
regulation as the ambient temperature changes.

To accomplish this, you want to add thermal capacitance, NOT thermal 
resistance.  The Dewar adds thermal resistance.

My solution is to seal the OCXO up in a fairly heavy cast aluminum 
box (I put the OCXO on teflon or nylon standoffs so there is no 
direct metal heat-conducting path from the OCXO to the box).  You 
don't really need to do anything more than this for most OCXOs.  If 
you want, you can mount the cast box in another enclosure (again, on 
thermally-insulating standoffs) with a thermostatically-controlled 
fan.  When I do this, I bond a thermal sensor to the cast aluminum 
box and use that to drive the fan, so the cast box remains at a 
constant temperature regardless of changes in the ambient temperature.

Best regards,

Charles