[time-nuts] Insulation - was: OCXO and Aging

[Will Matney]

Bob,

It's according to what other components are mounted with the OCXO. I have
seen HP OCXO's, that are just the metal can type, mounted within a Styrofoam
enclosure, with no harmful effects. Now, if there is a board with it made to
be mounted in the free air, or where air is circulated by a fan, then the
elevated temperature may harm some of the parts, if their operating
temperature is exceeded. Mostly, what would be worrisome is electrolytic
caps, as ceramic, mica, etc. shouldn't be hurt too much by that amount of
heat. Electrolytic caps have an operating temp. range, and one would need to
look at that. Resistors, you wouldn't need to worry about, but some
semiconductors, especially chips, that need cooling might get overheated.

Anyhow, the OCXO's that are made solely within a metal can, can be operated
this way, since the can and internal insulation traps the heat inside
anyhow.

On the test you ran, I think if an internal core temperature was read at the
crystal, you would find it around 118 degrees all the time. The 105 degree
temperature is what one would read after the ambient air has cooled off the
case/can somewhat, but it would be hotter on the other side of the heater,
towards the crystal, as the OCXO's internal insulation would be between the
heater and the outer can. That's my guess at it, but the Styrofoam
insulation just kept the whole thing at, or close to, the internal operating
temperature.

Will

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
Behalf Of Bob Smither
Sent: Thursday, September 15, 2011 12:36 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Insulation - was: OCXO and Aging


Will Matney wrote:
> Paul,
>
> The info they show is generally from the manual or paperwork that comes
with
> the OCXO when new.
>
> If they are used OCXO's, then they will be more stable over time than a
new
> one, off the shelf, as far as drift, or they should be. The OCXO acts
> similar to any heated device, and resistors function similar to this also.
> When first used, they will drift from spec more than they do after they
have
> ran for so long in time. The factory generally does a burn in to stabilize
> them, but it still takes some time after that to completely settle down to
> where calibration isn't needed as often. There's generally a curve for
each
> model that will show the drift over time.
>
> The main thing in using these, even though they're supposed to be
insulated,
> is to mount them where temperature fluctuations are minimal. Also, some
are
> better insulated than others, like the HP units, which are encased in foam
> inside a can.

<snip>

I scanned the archives for this topic, so if I missed it and it has
already been beat to death, please point me to any previous discussions.

What is the list's opinion about putting OCXOs in an insulated
enclosure?  That should reduce the effect of drafts in the lab.  I have
some insulin shipping boxes and put an OCXO in one for a few days.  The
OCXO is an C-MAC STP2145A.  I measured the current draw of the OCXO and
the metal case temperature with the lid of the insulated box on and off:

Lid off: Is ~ 163 mA, Tc ~ 105F
Lid on : Is ~ 130 mA, Tc ~ 118F

I assume the oven operates at the same temperature in both cases.
Clearly there is less power needed as less heat is escaping.  It looks
like the other circuit elements (outside the oven) are operating at
higher temperatures - likely not a good thing.

My measuring capability is limited.  Within the precision (~10E-8) that
I have I could not see any difference in the oscillator frequency.

Thanks,

Bob Smither



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