[time-nuts] Thunderbolt stability and ambient temperature

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jun 10 17:36:24 EDT 2009


Joseph M Gwinn skrev:
> time-nuts-bounces at febo.com wrote on 06/10/2009 03:13:19 AM:
> 
>> In message <4A2EFC6D.4020205 at xtra.co.nz>, Bruce Griffiths writes:
>>
>> Bruce,
>>
>>>>> The thermal time constant (not the thermal impedance per se) is what
>>>>> matters [...]
>> That is pretty much exactly the (mis-)definition of thermal impedance.
>>
>> Thermal timeconstant or thermal corner-frequency had been much 
>> better names.
>>
>>>>> It is possible to construct an enclosure with a long thermal time
>>>>> constant together with relatively low thermal resistance so that the
>>>>> temperature of a GPSDO or similar device within the enclosure only
>>>>> increases by a relatively small amount.
>>>> Nope. This is essentially a thermal low pass filter.
>> Well, yes you can, but it is not very useful:
>>
>> A really huge block of metal will do that:  It can transfer a lot
>> of heat (=low resistance), but will take a long time doing so (=high
>> impedance).
> 
> I read somewhere the suggestion to take the cast iron block from an old 
> automobile engine and put it in a heavy insulated wooden cabinet: ~250 Kg 
> of iron in an insulated box.  Cracked blocks are useless in an engine, and 
> so are available in junkyards quite cheap.

It is still just approaching a 1 pole filter. Alternating 
insulation/low-capacitivity - conduction/high-capacitivity in several 
layers creates higher pole systems.

The alternation in impedance creates impedance missmatches and we should 
expect reflections...

Oh, an interesting experiment is to try to freeze a watermelon in a 
steaming hot desert. Put towels on it and poor water over it. The amount 
of energy needed to evaporize the water exceeds the suns heatup effect, 
so the watermelon cools down... down... down... a bit of non-linearity 
causes what may seem to be a thermal induction effect.

Cheers,
Magnus




More information about the time-nuts mailing list