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

[Bob Camp]

Hi

What you have in the LTE is a TCXO rather than a bare crystal or an OCXO. It’s got a compensation circuit that corrects the FT curve of the crystal. The net result is likely a 5th or higher order curve when you plot frequency over temperature. Every TCXO off that production line will have a different curve. You would need a full characterization of that curve for your specific TCXO to pick an optimum point. 

With a GPSDO, taking care of the long term drift is not what you are after. The GPS does that. If the GPSDO is TCXO based, the the loop filter is going to be pretty fast. That is *not* a knock on the LTE part, it’s just physics. An OCXO part is a different beast. Each has their strong points. Don’t try to run the OCXO off batteries for a week … With a fast filter, temperature variations at the “per hour” level are not likely an issue.

Once you get to the point that drafts are worked out, and that temperature change is slowed down, you are done. No need for anything more complex.

Bob

> On Nov 23, 2014, at 9:30 AM, Jim Sanford <wb4gcs at wb4gcs.org> wrote:
> 
> All:
> I am enjoying this thread.  These are all very interesting ideas.
> 
> Hoping to power up my first unit later today....
> 
> I'm putting my LTE-Lite in the recommended HAMMOND box.  That takes care of the box with air.  I was then considering proportional heating of the surface of the box, like I did long ago with some GUNNPLEXERS -- seemed to work pretty well.   Then this whole assembly goes inside two or four inches of the foam insulation.
> 
> Now, the question becomes, to what temperature to heat it?  With a crystal, I'd plot /f/ vs. /T/, and look for minimum slope.  How to do that with LTE-Lite -- plot /efc/ vs /T/ and look for either center of range or minimum slope??
> 
> Thoughts?
> 
> Jim
> wb4gcs at amsat.org
> 
> On 11/23/2014 9:03 AM, Magnus Danielson 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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