[time-nuts] Achievable temperature stability for Thunderbolt environment?
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Sat Jan 15 19:59:00 UTC 2011
Achim wrote:
>I am just wondering, if I should rather worry more about high
>temperature gradients rather than excursions from a mean value, as
>slow variation can be compensated by the control loop while for
>quick changes, the loop is just too slow :)
This has been discussed quite a bit on the list, so you will find
information in the archives. The current version of Lady Heather
facilitates adding overall temperature regulation, if you so
desire. Many of us have found that excellent performance can be
achieved simply by ensuring that changes to the Tbolt's enclosure
teperature are filtered by a longish time constant.
First, note that the reported temperature is measured at the
periphery of the Tbolt pc card, not inside the crystal oven. Second,
the GPS disciplines the oscillator in a high-gain loop through a time
constant and filter Q set in software (which can be determined by the
user), typically from 200-2000 seconds. For best performance, this
time constant will be set so that it allows the GPS to control the
oscillator at longer averaging periods (tau), where the oscillator's
drift reaches its "random walk" phase, but leaves the crystal
substantially to its own devices at shorter tau.
Assuming sufficiently high gain in the disciplining loop, one only
needs to make the thermal time constant between the Tbolt pc card and
ambient temperature significantly longer than the control loop for
the control loop to handle temperature changes as well as random
oscillator drift. This can be accomplished by housing the Tbolt in a
somewhat larger enclosure. Note that insulating it too well from
ambient temperature (resistive loss) is not desirable because it
allows too much of a temperature rise and thereby deprives the oven
control loop of the temperature sinking it needs to operate properly,
as well as subjecting the components outside the oven to
unnecessarily high temperatures that could tend to create reliability
problems. In my tests, reported temperatures up to 45 degrees C work
well. What you are after is thermal reactance to slow down the
change experienced by the Tbolt, not so much thermal resistance.
I settled on a cast aluminum project box that provides about 1" (25.4
mm) of clearance around the Tbolt and has 1" rubber feet to minimize
conductive heat transfer to the environment. I mount the Tbolt on
standoffs (I used nylon standoffs to minimize thermal conduction, but
I suspect metal standoffs would be fine). One can then ask whether
the convection currents within this outer enclosure work against
you. I have tried (i) adding a small fan to circulate the air within
the enclosure, and (ii) adding foam with large open cells to disrupt
the convection. I could detect no difference other than another 0.75
degree C rise with the foam, so I returned to a simple air space.
In my case, the outer enclosure is mounted inside a rack-mount
enclosure that also contains buffers, dividers, and signal
conditioners to provide the various reference signals I need. There
is a minimal temperature rise inside this box -- it is cooled with a
very slow fan. Overall, I get about a 1 degree C swing over 24 hours
as reported by the Tbolt temperature sensor, superimposed on a 2 or 3
degree C semi-annual swing. There is very little change in the
temperature data at time scales below several hours.
Best regards,
Charles
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