[time-nuts] Thunderbolt stability and ambient temperature

Predrag Dukic stijena at tapko.de
Thu Jun 11 08:17:25 EDT 2009






I didn't try it but if You want to average daily (or even seasonal) 
temperature changes,

one could drill the floor of  a cellar ( Your time-nuts gear is in 
the cellar, isn't it?)

with the geotechnical drill used for core samples of soil and rock .

3-4 inch diameter drills are standard, and surely make enough space for 10811.

Few meters down, I expect seasonal changes 1-2 deg  with 50 deg 
external winter-summer change.

Under buildings probably even less...(permafrost story...)

Plastic sewage pipe inserted in to keep water out, and You 
have  thermal mass as much as You like.





P. Dukic










At 12:46 11.6.2009, you wrote:
> > For PLL steered devices, you want your device enclosed by a
> > thermal mass which is again enclosed by a layer of thermal
> > isolation.  The goal is to filter/average all rapid (daily ?)
> > external temperature influences, only letting through such
> > slow variations (seasonal ?) which the PLL can comfortably cope with.
>
>My impression with this thread is that two sligthly different things are
>being discussed here:
>
>Some posters explain how a crystal oven shall be designed as Poul-Henning
>does with his words above. Absolute correct, no doubt about it!
>
>Nevertheless only a VERY few of us (Rick for example) have real experience
>in designing ovens and most of us are confronted with a existent ready to go
>OCXOs. That is why one can start a discussion about the question whether the
>thermal properties of an existing OCXO can be improved which is what the
>second group of posters has done starting with the Thunderbolt.
>
>The situation with the ready to go OCXO is a bit different than starting
>from scratch: The OCXO DOES already have a certain thermal mass inside and
>it DOES already have a certain thermal insulation of this mass against the
>ambient.
>
>BOTH of these parameters have been important properties for the designer of
>the oven's temperature controller. The temperature controller's properties
>must match EXACTLY the thermal mass as well as the amount of insulation in
>order to work as expected. If one of you has personal experience with high
>precision temperature controllers I am sure he can second this claim. The
>situation gets the harder the better the thermal insulation is: You need to
>avoid every kind of regulation overshot because it is only the high
>insulation that allows energy to leak out of the thermal mass. Which in turn
>requires high regulator time constants.
>
>By the way: That is also the reason for the heat sinks on rubidium
>standards. They form a good coupling to the ambient (=small insulation)
>making the design of the lamp's temperature controller much more easy and
>much faster than with a good insulation.
>
>With a ready OCXO in your hand like the 10811 or the FTS1200 or anything
>else: You can't improve the beast by "simply better insulation" or by
>"simply higher thermal mass", because every change in this will have worse
>impacts on the action of the temperature controller.
>
>However, there ARE ways to improve the thermal behavour of an ready to go
>OCXO:
>
>First rule: Allow the temperature controller to "see" the original thermal
>mass, i.e. don't open the enclosure and don't change anything, just leave
>everything as it is.
>
>Second rule: Allow the temperature controller to "see" the original thermal
>insulation, i.e. allow for some cubic centimeters of air around the OCXO's
>outer enclosure. It was engineered with that surrounding in mind!
>
>Third rule: Outside of the these few cubic centimeters of air around the
>OCXO you are allowed ANYTHING.
>
>You can for example put a big thermal mass like a massive aluminium
>enclosure around the OCXO (but please don't forget the few cubic centimeters
>of air between this and the surface of the OCXO). THIS thermal mass in
>conjunction with the air around it will work as the thermal lowpass filter
>that some posters talked about but WITHOUT worrying the OCXO's temperature
>controller because it "sees" the original thermal mass and the original
>thermal insulation. But it "sees" smaller ambient temperature variations
>than before.
>
>This would be a passive solution where the aim were to get high time
>constants for the combination of the outer enclosure and the surrounding air
>making as much thermal mass as possible the prefered way. One of my 10811
>resides in a 12 X 12 X 12 cm aluminium enclosure with 2 cm wall thickness.
>This has given a thermal time constant in the order of precious few hours if
>i remember correctly and was far from averaging daily temperature changes.
>It might however be helpful for the discussed problem of Thunderbolt
>reactions to short time temperature changes.
>
>Or one can decide for a active solution. This would call for an outer
>enclosure where not high thermal mass but high thermal conductivity were the
>aim which is brought to a constant temperature by means of a second
>temperature controller, the famous "double oven" principle if you like. As
>in the passive case there must be enough air between OCXO and the outer
>controller in order to avoid any interaction between the temperature
>controllers.
>
>Best regards
>Ulrich Bangert
>
>
>
> > -----Ursprungliche Nachricht-----
> > Von: time-nuts-bounces at febo.com
> > [mailto:time-nuts-bounces at febo.com] Im Auftrag von Poul-Henning Kamp
> > Gesendet: Donnerstag, 11. Juni 2009 00:43
> > An: Discussion of precise time and frequency measurement
> > Betreff: Re: [time-nuts] Thunderbolt stability and ambient temperature
> >
> >
> > In message <4A3033FC.6070806 at xtra.co.nz>, Bruce Griffiths writes:
> > >Hal Murray wrote:
> >
> > >Thermal wave reflection at boundaries/interfaces does occur:
> >
> > If your frequency or voltage standard is in a physical
> > environment where these effects are relevant, you have much
> > bigger problems to deal with before thermal reflection
> > becomes your number one priority :-)
> >
> > Lets stay real here.
> >
> > Summary:
> >
> > For PLL steered devices, you want your device enclosed by a
> > thermal mass which is again enclosed by a layer of thermal
> > isolation.  The goal is to filter/average all rapid (daily ?)
> > external temperature influences, only letting through such
> > slow variations (seasonal ?) which the PLL can comfortably cope with.
> >
> > Even if you use active temperature control, peltier or
> > otherwise, it is still a good idea to employ a thermal mass
> > to cope gracefully with power-failuers and other equipment glitches.
> >
> > Poul-Henning
> >
> > PS: The thermal mass need not be solid blocks of metal,
> > regular ceramic bricks or tiles work fine.
> >
> > --
> > 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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