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

[Magnus Danielson]

Hi David,

We did a fairly simple measurement setup at work.

We had the oscillator sitting on a small test-board and measured the 
frequency from start. Then a few seconds in we shifted the direction of 
a fan at some distance onto the oscillator. We then did this with a 
variation of simple shields, and concluded that a fairly simple wind 
shield achieved most of the gains we where after. We then reapplied this 
in various incarnations since, and it has not provided us with any 
reasons to do things differently, but rather once the lesson was 
learned, it was shown effective in many places, as forces convection is 
an unfortunate needed thing in our products.

As most oscillators have a metal can, they conduct heat well and if 
there is no direct forced air convection onto it, it allows the 
radiation and still air conduction to be fairly well evened out and 
those provide less temperature gradients to the oscillator.

I've also seen the 5370A/B shield. It works and solves the problem, but 
often you can use simpler setups with good results too.

So, it comes down to not really shielding it from long-term temperature 
variations, but just not make the situation much worse than it needs to be.

If one has a box with relatively low power consumption per unit volume, 
forced air is not needed, and need for shielding can be relaxed. Just 
putting the oscillator of from heat sources and in particular heat 
sources that vary over time come far. The important part is that it is 
in a thermally quiet corner, which include air and air-flows.

We had a pair of students doing work during summer vacation. They where 
measuring the phase stability of one of our boxes. Three hours into the 
measurements the variations seemed to go away. They where completely 
puzzled. It was showing clear variations and then the systematic died 
away. So I just asked them when they started the measurement. "Around 
15:00", well that was all I needed. I informed them that the building AC 
turned off at 18:00, and what they was measuring was variations in 
ambient air condition. They where flabbergasted and wondered how it 
could have such an effect. So I pulled the board out of the chassi and 
showed them the oscillator location and showed them how the side-wise 
blowing air hit the can. I then found them some foam tape and advised 
them to apply it to the oscillator and redo the measurement. It was much 
flatter naturally. However, for that product it was good enough without 
the shielding, but we changed modus operandi for all our products since. 
For this very good reason.

Cheers,
Magnus

On 2022-07-01 22:31, Dr. David Kirkby via time-nuts wrote:
> On Fri, 1 Jul 2022 at 20:11, 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?
>>
> I removed an HP 10811A OCXO from a 5370B time interval counter the other
> day and put it into a HP 5352B 40 GHz frequency counter. One thing that
> really struck me is that in the 5370B there was a shroud around the OCXO,
> which is around 5 mm away from the sides of the OCXO. It's made of
> aluminium. But there's nothing like that in the frequency counter. The two
> attached photographs show a significant difference. I took the photograph
> from inside the 5352B frequency counter. The photo of the 5370B was one I
> just found on the EEV blog site, as I did not want to have to mess around
> taking another photograph.
>
> I see Magnus respond to you.
>
> My gut feeling is the designers of the 5370B were likely to have more
> knowledge about the behaviour of oscillators than the frequency counter
> designers, which makes me wonder if adding something around the oscillator
> in the frequency counter, like in the 5370B time-interval counter, might be
> a good idea.
>
> Unfortunately I suspect it would be very time-consuming to evaluate the
> difference a shield would make in the frequency counter, I have another HP
> frequency counter where the fan blows over the oven, which does not seem a
> very good idea.
>
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