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

Bob Camp kb8tq at n1k.org
Sun Nov 23 09:45:04 EST 2014


If you have a basement in your house / building 

—and —

it’s dry and reasonably draft free (no garage doors opening up from time to time)

— and —

At least one side / corner is well buried in the ground 

— and —

You can get at that corner / side.

Move your thermal baffle gizmo up against that wall, move it into that corner. There is a lot more mass in the foundation of a building than anything you would want to lug around for a project. You still need to handle the issues on at least half the surface, that should be less trouble than doing the whole thing. 

There is another subtle advantage to this approach. The standard is out of the way. It’s not in the middle of the lab. It does not get bumped. It does not get sparked (unless you have full ESD protection in the lab …). It’s less likely to have random power cycle events due to cords being accidentally pulled. Even second order stuff related to ground loops from connecting and disconnecting cables may be reduced.  “Just leave it alone” is much easier to do when the gizmo is surrounded by a pile of bricks.

With a GPSDO, you don’t care (much) about the environmental  swings from week to week or month to month. The GPS will take care of that. What you care about are the hour to hour or minute to minute  movements. Those are the ones that the filter on an OCXO based unit will struggle with.  Hotter in the summer / colder in the winter is not as big a deal as “cold when I come  in / hot after I turn everything on”. 

One practical hint if you do try this: 

Put a cheap plastic bag around the gizmo and tape it up. It discourages the bug colonies. I have empirical evidence that this is a good idea ...


> On Nov 23, 2014, at 9:03 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> NIST did something similar for their WWWV site, where they used bottled water in its staple packaging to build a thermal mass. They measured how their atomic clocks and rig behaved before and after, and could see the difference. Very neat way of using off the (store)shelf components for a test.
> Another aspect is to think about what kind of heating/coolling you have. If it can act more as a proportional system rather than bang-bang regulations, it won't produce as drastic swings for you.
> Cheers,
> Magnus
> On 11/23/2014 02:32 PM, Poul-Henning Kamp wrote:
>> --------
>> 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.
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