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

Charles Steinmetz csteinmetz at yandex.com
Sun Nov 23 07:37:35 EST 2014

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,


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