[time-nuts] Thunderbolt stability and ambient temperature

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jun 9 18:33:22 EDT 2009

Poul-Henning Kamp wrote:
> In message <BLU125-W9E23FDF3E1618C93E603ECE440 at phx.gbl>, Mark Sims writes:
>> I think that using a well insulated box [...]
> The point for timenuts is not just the thermal resistance, but more
> importantly the thermal *impedance*: you want to low-pass filter
> the thermal changes so that they all happen in the area where the
> PLL can cope with them.
> Thermal resistance is about insulation, thermal impedance is
> about (thermal) mass.
> So you significant mass and volume (like a fridge) not light and
> small (like a cardboard box).
> When metrology people really want to keep things at the same
> temperature, they mount them in oil-baths (for good thermal contact)
> in the middle of a block of aluminium, typically 2'x2'x1' (for
> thermal impedance) which is again insulated with 1" styrofoam, all
> of this mounted in a plywoodbox, set on rubber-wheels to get it off
> the floor (for thermal resistance).
> If they are really into this, they cover the plywood with high-quality
> (noble-) metal foil, to maximize reflectivity and minimize emissivity,
> so that the black-body radiation from devices and humans in the lab
> does not affect the temperature interface as much.
> Then they leave it alone for "some weeks" in their temperature
> controlled lab so the temperature can stabilize.
> At this point they may start to wonder how they can verify the pt100
> temperature sensor they put in the middle of it all actually works
> when the temperature never changes...
> Poul-Henning

The thermal time constant (not the thermal impedance per se) is what
matters when one is trying to reduce the effective amplitude of
temperature fluctuations due to air conditioner cycling.
Adding mass increases the thermal capacitance adding insulation
increases the thermal resistance.
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.

Multiple alternating layers of thermal conductor and thermal insulator
reduce thermal gradients as well as temperature fluctuations.

Having an outer conductive layer reduces the temperature gradients over
the insulator surface.

Readily available inexpensive aluminium foil is a cheaper alternative to
expensive noble metal foils.

Silica aerogel is one of the most effective insulators.

Balsa wood has been used as the insulator in portable temperature
controlled ensclosures for standard cells.


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