[time-nuts] Thunderbolt stability and ambient temperature

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jun 10 04:15:37 EDT 2009


Poul-Henning Kamp skrev:
> 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.

Thermal impedances can be divided into thermal resistance and thermal 
capacitivity, these forms filters which in a 1D representation would be 
a number of RC-links of various values.

Thermal time constant and total thermal conductance is aspects of the 
static and dynamic properties that the structure has.

>>>> 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).

Depends on the metal.

>> Adding just a little insulation to the added thermal mass can
>> dramatically increase the thermal time constant combined with a modest
>> increase in operating temperature.
> 
> Isn't that exactly what I explained initially ?
> 
> A huge block of aluminium, encased in 1" of styrofoam ?

There are many ways to say the same thing, don't argue with the guy just 
phrasing it differently. Discuss the various merits of different 
approaches instead.

>>>> Readily available inexpensive aluminium foil is a cheaper alternative to
>>>> expensive noble metal foils.
> 
> But it does not stay as reflective.  By the time you add this layer to
> the construction I mentioned, you care about the difference between
> 0.98 and 0.99.

It is interesting to see you being more extreme then Bruce. At the same 
time, you are discussing two different approaches, not a single one.

I think discussing a general hints and tips that can apply to many 
pratical cases is more useful at this stage then discussing a detailed 
extreme solution only.

Nobody having a thermal inductive material around? That would be very 
usefull to handle temperature shifts.

Cheers,
Magnus




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