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

[Mike Garvey]

We should not forget that the LTE is not a free standing frequency source:
it is steered to GPS signals and, in the long term, will reflect the
accuracy of GPS and the stability of the as-received signal.
>From the ADEV plot in the user manual, it looks like the time constant for
steering of the TCXO is about one minute; constructing an enclosure with a
time constant significant longer than a few minutes has diminishing returns.
Mike

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of
Poul-Henning Kamp
Sent: Sunday, November 23, 2014 8:33 AM
To: Discussion of precise time and frequency measurement; Charles Steinmetz
Subject: Re: [time-nuts] Practical considerations making a lab standard with
an LTE lite

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

>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
temperature.)

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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