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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
