Hi I believe that if you go back a few years in the archives, you will find a thread that ultimately stops with a swimming pool full of mercury. Needless to say, we’re been down this road once or twice before.
Bob > On Nov 23, 2014, at 7:59 PM, Neville Michie <namic...@gmail.com> wrote: > > A Hint about avoiding convective cell heat transfer, > If you keep the spacing between two planes less than 5/16" then you will > be unlikely to have convection cells forming. The stationary air is a good > insulator > but thermal radiation will be the dominant heat transfer process. > This is true for double glazing, katharometers and generally all devices. > The suppression of turbulent heat transfer may provide more insulation but > also > less noise and instability. > So it may be a good idea to use a relatively close fitting box with thick > walls. > Cheers, > Neville Michie > > > > > On 23/11/2014, at 11:37 PM, Charles Steinmetz wrote: > >> Dave wrote >> >>> But given the TCXO"s sensitivity to temperature changes, I don't >>> know whether it might be preferable to mount the LTE lite in its own box >>> without any power supplies in it - perhaps with some thermally insulting >>> material around the LTE lite so the crystal doesn't experience any fast >>> temperature changes. >> >> First, mount the LTE in a cast aluminum box (not thin sheet metal, something >> with some heft). Use thermally insulating standoffs (teflon or nylon, with >> no metal "through" fasteners) to get the board in the middle of the volume >> of the box. Use a box a bit larger than you'd first think, so there is at >> least 1" of air on all 6 sides of the LTE board. Do NOT mount any part of >> the LTE board (connectors, etc.) directly to the box walls -- use "pigtails" >> for all connections. Do NOT use any insulation between the LTE and the box >> walls other than the 1"+ of air. >> >> The mounting described above will add substantial thermal capacitance to the >> LTE board (good) without adding significant thermal resistance (bad). For >> further discussions of this issue, search the list archives for "thermal >> capacitance" and "thermal mass." >> >> Now, mount the cast box (plus any thermal mass you add to it -- see below) >> so that IT is thermally isolated from the overall enclosure (or, if it sits >> out in the open, thermally isolated from anything solid). The air space in >> the enclosure isolates the oscillator from the cast box and the box is >> sufficiently massive that its temperature cannot change nearly as fast as >> ambient. The thermal mass of the cast box can be adjusted by adding thermal >> mass to it as desired. >> >> The goal is for the box temperature to change only by changes in ambient AIR >> temperature, and the LTE board to change only by changes in the AIR >> temperature inside the cast box. This integrates any changes to the LTE >> board temperature with a very long time constant, which allows the GPS >> discipline to track and cancel the temperature changes. >> >> (If you mount an ovenized oscillator this same way, it integrates any >> changes to the OCXO temperature so that the oven control loop can track and >> cancel any changes to the crystal temperature.) >> >> You can, of course, improve things even further by making sure the ambient >> air temperature surrounding the cast box changes slowly, or not at all. But >> the technique described above can be counted on to reduce thermal effects in >> most OCXOs or GPSDOs to better (often much better) than the 1e-13 level >> unless the ambient temperature changes MUCH more and MUCH faster than any >> change we wouild consider normal for a living space. This is true whether >> the cast box is mounted out in the open, or inside an overall enclosure with >> other electronics. >> >> Best regards, >> >> Charles >> >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
