[time-nuts] Practical considerations making a lab standard with an LTE lite

Neville Michie namichie at gmail.com
Sun Nov 23 19:59:47 EST 2014

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.
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
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