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

[Bob kb8tq]

Hi

If you tear into *lots* of HP devices with OCXO’s in them (not just the 10811 version),
the most typical place for the OCXO is right. next to the power supply. That puts it inline
with the output of the fan. 

Why? The OCXO gets hot. Heat buildup in the instrument is not a good thing. They put
it there to get the heat out of the box as quickly as they can. Since the “draft” is a constant
(and not a puff puff puff) it’s not as big a deal as you might think. What it does do is to
move the changes in the *outside* environment over to the OCXO more quickly.  If there
is an impact, it is from your lab ….

Bob

> On Jul 1, 2022, at 12:31 PM, Dr. David Kirkby via time-nuts <time-nuts at lists.febo.com> 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.
> <IMG_20181128_190550small.jpg><OCXO-in-frequency-counter2.jpg>_______________________________________________
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