[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?

[Magnus Danielson]

Hi Eric,

On 2022-07-01 16:40, Erik Kaashoek via time-nuts wrote:
> I'm trying to build a stable reference for a phase noise meter project 
> and have acquired a double oven XO that boosts high short term 
> stability (below 1e-12/s). But the spec also states that, even with 
> the double oven, there is still substantial impact of environmental 
> temperature changes (below 1e-8 changes over the normal operating 
> temperature range) so I was wandering if its good practice to try to 
> thermally isolate the DOCXO or do you run the risk of overheating as 
> it always may burn some power and its better to only shield it from 
> draft?

You should be careful to isolate it too much.

OK, let's get the basics. The oven aims to maintain a certain 
temperature by running a heater continuously and balance the heating to 
the cooling of the surrounding, as it dissapates heat. This is equally 
true for double and tripple ovens, they just have different temperature 
settings.

Now, if you over-isolate any oven, the heat transfer will be too low so 
the heater will overshoot the heating. When this happens, the heater 
turn fully off and the oven will coast down unregulated until low enough 
temperature. What you then end up with is a bang-bang regulator causing 
a saw-tooth like heating profile. This is then worse situation than before.

Naturally, this all depends on the design of the oven, and how it's 
setpoints is done, but the ambient temperature specification gives the 
clue of how far you can go. You need to remain the thermal loading to 
maintain that minimum heat conduction out of the oven. For passive you 
need to respect the highest ambient temperature (of the oven) for all 
ambient temperature conditions of the device you build. Isolation needs 
to be done carefully, and passive stability is hard. Active measures 
naturally can help but you then need to handle cooling.

Rather than thinking isolation, you should rather avoid direct 
variations of forces convection air path. Essentially, put the OCXO in a 
draft-free corner. Essentially wind-shielding it but really not doing 
any actual isolation to maintain heat conduction away from the OCXO 
works really well. Either just a few metal walls or a plastic cap around 
it will suffice to cause much of the effect without excess danger of 
over-isolation.

For test-purposes, you find that I often put oscillators inside a 
cardboard box with some antistatic bubble-wrap around it. Not enough 
thermal mass for long-term things, but good enough to remove much of the 
quicker fluctuations. There is usually an ADEV bump at 500-1500 s 
traceable to the heating/AC. Similarly a beach towel have surved the 
purpose for larger things.

Cheers,
Magnus

> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe send an email to time-nuts-leave at lists.febo.com