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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
