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

[Bob kb8tq]

Hi

Modern OCXO’s are set up based on temperature run data. 
They play with set point (and possibly electrical gain) to optimize
the TC contribution of the crystal *and* the rest of the parts in
the device. This is true of single and double ovens. 

One not so obvious point is that these runs are done in a very
specific temperature environment. Forced air is normally part of 
a test chamber. It also is often part of the OEM installation that 
the OCXO goes into. 

Change the “air pattern” to much and you change the gain of the
oven ( more insulation increases the gain). This can upset the careful
balance done when optimizing the TC of the device. 

One can debate just how stable all of the “stuff” in an OCXO is over
the years. Is a set ( or screening ) done on a production line a decade
ago still relevant today? Random bits of evidence suggest that the TC
optimization holds pretty well, but there isn’t a lot of data. 

A typical double oven should be < 5x10^-10 over 0 to 70C. Indeed 
many manufacturers will sell you examples that are spec’d tighter 
than that. Some offer single ovens with spec’s below 1 ppb over 0 to
70. 1x10^-8 is a very typical single oven spec. 

How well does this or that example do? It is not uncommon to see 
1x10^-8 level single ovens rolling off the production line at <2x10^-9.
On some designs > 80% of the units do this. Counting on any and 
every OCXO to be 5X better than spec …. maybe not, but many designs
do. 

How to “manage” an OCXO? 

First step it to get a good one in the first place. If eBay is your source of 
supply ( it is for me ….) what you get likely is not going to be 100% perfect. 
Some level of testing and sorting will be involved. That needs to be done 
before a lot of additional effort is put in. 

Next up is to plan on keeping it on power all the time. OCXO’s don’t like
to be cycled. Sorry about that. If this bugs you, don’t head down this 
road. There are good reasons for this to bug you so do think about it.

Drafts and abrupt temperature changes are to be avoided. Opening the 
lab window next to your reference standard … not a great idea. Something
as simple as a towel or a cardboard box tossed over the device can do
wonders. Exotic enclosures are probably better, but simple gets you a 
long way. Thermal mass might help as well. 

Just as a note, things like Rb standards (and Masers) also are said to 
benefit from fairly simple “draft protection” enclosures. 

Most folks are pretty obsessive about regulated supplies. If anything they
go a bit overboard in terms of noise for an OCXO supply. What might get
overlooked is the need for a fairly substantial ( = low voltage drop) supply
wiring setup (along with good ground practices). If you plan some sort
of battery backup, consider the regulation impact as it cuts in and out. 

Loading on the output of an OCXO does matter. How much is a “design 
feature”. It is not uncommon to see a few minutes of disruption for a 
significant load change. Simple answer here is not to play with moving 
things around a lot :) 

Many OCXO’s are tuned via an EFC. Feeding this input in a stable fashion
can get a bit crazy. Do try to run the EFC circuit ground straight back to
the OCXO. Oven current induced drops are not great for EFC stability ….

Fun !!!

Bob

> On Jul 1, 2022, at 6:40 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> 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?
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