Charles
It's not practicable for many people but - what about replacing your
aluminium box with a, say 2 foot piece of 6 inch pvc pipe (ocxo suspended
inside it clear of the wall and sealed off ends) and burying that a few feet
in the ground? I expect the thermal mass of the pvc is not as high as the
aluminium but the external ground temperature will be lower than air ambient
as well as having much less variation. Overall, I'm not sure what the result
might be.
DaveB, NZ
----- Original Message -----
From: "Charles Steinmetz" <csteinm...@yandex.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Thursday, January 30, 2014 9:28 AM
Subject: Re: [time-nuts] Morion MV89A position
/tvb wrote:
I tried something like that and ended up frying the oscillator! The
problem was that I used too much insulation. That was good to keep ambient
temperature transients out but it also kept internal heat generation in.
* * *
My question is, how does one design an enclosure to prevent this mistake?
Or is it trial and error. Perhaps put a thermistor on the OCXO and if the
case temperature rises beyond what is normal case temperature in free air,
then the enclosure has too much insulation?
That is the point of distinguishing thermal capacitance (= thermal mass)
from thermal resistance. Generally, OCXOs are thermally designed to be
"naked" in some abient environment that is close to room temperature
(usually somewhat above, anticipating they will be installed in warm
instruments and not be sitting naked on a desktop). They will work
acceptably in an ambient environment ranging some tens of degrees C above
and below this, as long as the ambient temperature does not change too
fast for the oven controller to keep up (for example, the HP 10811 is
rated for -55C to 71C, with degraded temperature stability below 0C).
Adding additional thermal resistance (your insulation, or Paul's Dewar)
unbalances the heat flow to ambient. In theory, the oscillator should
respond by backing down the heater current to match heater power to the
new thermal resistance. However, raising the thermal resistance can
destabilize the thermal control loops in some OCXOs. When that happens,
you get poor thermal regulation at best and you burn down the oscillator
at worst. (And even if the control loop does not destabilize, it won't be
running at the design center "sweet spot" foreseen by the designers, so
thermal stability will likely be degraded.)
What you want to do is keep the net thermal *resistance* similar to what
it is with the oscillator in its target environment, and add thermal
*capacitance* to slow down the changes in the oscillator's immediate
environment (i.e., just outside the oscillator can). The metal box does
just that. The thermally resistive path is essentially still all air,
just as it is with the oscillator sitting on a desktop -- from the
oscillator through the air surrounding it to the inside wall of the cast
box, then from the outside wall of the cast box through the air to ambient
(to a first approximation, the temperature of the inside wall of the box
is the same as the temperature of the outside wall). This way, the
average thermal resistance from the OCXO to ambient is still similar to
what the thermal designers contemplated. However, the cast box averages
(integrates) the outside temperature as seen by the OCXO with a time
constant measured in tens of minutes.
The net result is that the oscillator is in (or very close to) its design
environment with respect to the average heat loss, so the oven controller
is working at (or very near) its design sweet spot -- but at the same
time, the rate of temperature change seen by the OCXO (i.e., its immediate
environment inside the cast box) is integrated over tens of minutes. If
the ambient temperature did not change at all, the OCXO's immediate
environment inside the box would be exactly the same as if it were not in
the box but, rather, sitting naked on a desktop. When the ambient
temperature changes fast, the OCXO's immediate environment follows it --
but much more slowly, giving the oven control loop time to adjust to the
change without introducing a transient error.
As long as you end up with sufficient thermal capacitance to slow all
expected thermal transients down to the point that the oven control loop
has no trouble keeping the quartz crystal in an isothermal state, you have
done all you need to do. There is nothing to calculate, and no
complicated design procedure. If the oscillator still exhibits some
transient thermal error, just increase the mass of the box by mounting
aluminum plates to the outside walls of the cast box until it doesn't.
The one exception to this would be if the oven controller simply has too
little gain to keep the crystal isothermal, in which case there would be a
DC (not transient) error (each ambient temperature would be associated
with a unique crystal temperature). Note that this would not be the fault
of the method -- rather, it would be a matter of poor regulation by the
OCXO's oven controller. It could be mitigated by active regulation of the
cast box temperature using an outer enclosure with a thermostatically
controlled fan.
Best regards,
Charles
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