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

Poul-Henning Kamp phk at phk.freebsd.dk
Sun Nov 23 08:32:36 EST 2014

In message <20141123153744.bioKftA5 at smtp16.mail.yandex.net>, Charles Steinmetz 

>First, mount the LTE in a cast aluminum box (not thin sheet metal, 
>something with some heft). [...]

Charles' design has some good points, but I don't agree with it.

What you are trying to do is to low-pass filter any thermal signals
before they reach the LTE or OCXO.

Charles' design works great from the outside, but doesn't do anything
with respect to the thermal energy expended by the encapsulated
device themselves, which will cause convection in the inner box.

(For LTE and OCXO it is probably less of a problem that changing
power-disipation will have a outsized effect on the central

Here is a much simpler and likely cheaper way to do it:

Put the LTE or OCXO in a small box of your choice.  Even a cardboard
box is fine.  A little thermal insulation in the box is OK, but not
too much, the heat must be able to get out.

Find a medium sized cardboard box, something like a cubic feet or so.

Place it where you want your house-standard, with some kind of
thermal insulation under it, two layers of old rug will do fine.

Lay a floor of bricks inside the box.

Build a "wall" of bricks along the outside of the box.

Place the smaller box in the hole in the middle, cut the
corner of a brick to run the cables without too much leakage.

Use a floortile as roof, possibly with a layer of bricks on top.

Close the outher cardboard box with tape to minimize convection.

Congratulations, you now have a cheap and incredibly efficient
thermal low-pas filter, which will allow thermal energy to move in
both directions -- eventually.

The outher cardboard box is not optional, unless you replace it
with some other "mostly air-tight" barrier.

The little bit of insulation the outher cardboard adds are not a
bad idea either, for instance it reduces the effect of sunlight
hits the box at certain times of the day/year.

But you can substitute any geological building material you have
at hand for the bricks, because the trick is that geological building
materials have just the right thermal properties we are looking
for:  Decent but not too good thermal conductivity with healthy
dose of thermal mass.

Cinderblocks comes with convenient interior holes premade.

Aerated concrete blocks are also a candidate material but
don't make it too thick since it insulates quite well, and
paint the surface to bind the dust.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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