[time-nuts] Thunderbolt stability and ambient temperature

J. Forster jfor at quik.com
Tue Jun 9 20:36:10 EDT 2009


> J. Forster wrote:
>>>> 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.
>>>>
>>
>> Not really.
>>
>> Consider a one dimensional model, with the controlled space onb the left
>> and the ambient on the right:
>>
>>      |------|           |
>>      | Osc  |===========| Ambient
>>      |======|           |
>>       T mass
>>
>>
>>> 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.
>>>
>>
>> True
>>
>>
>>> 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. The same resistance
>> enters into the time constant and thermal resistance to the ambient. The
>> only way to increase the thermal TC, while maintaining the resistance,
>> is
>> to add thermal mass to the Oscillator assembly. (Increase the C with
>> constant R)
>>
>> -John
>>
>>
> Incorrect assumption, I was discussing the thermal characteristics of
> the added enclosure.
> The thermal resistance of the external enclosure to the ambient isn't
> fixed.
>
> Just adding mass to the case without ensuring that the additional mass
> has a well defined (and not too low) thermal resistance to ambient can
> be relatively ineffective.

I never suggested adding thermal mass to the case, but to the oscillator
package to lengthen the thermal TC. (Tmass above)

> 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.

Adding insulation between the oscillator and the ambient will lengthen the
TC but also increase the temperature rise.


>>> 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.
>>>
>>> Bruce
>>>
>>
>>
>>
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>>
>
> Bruce
>
>






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