> 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 >>> >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > Bruce > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
