By having only one sensor, in the case of the double oven that is obviously not true, but what I meant was that even with a double oven, you only have one sensor near the components you try to regulate, the other sensor regulates the outside temperature of the first oven.
Didier > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Didier Juges > Sent: Saturday, July 12, 2008 8:50 AM > To: 'Discussion of precise time and frequency measurement' > Subject: Re: [time-nuts] Double ovened 10811-60158 on ebay > > My understanding of thermal regulation is that one of the > more difficult thing to do is precisely to know what is the > temperature of the components you are trying to stabilize. > > If you have one oven (even double oven), you have one > temperature sensor. > The best you can do is keep the sensor at a constant > temperature. Assuming the temperature sensor is kept at a > perfectly stable temperature, how constant will the > temperature of other components be will be a matter of > hardware design and layout. I.e. the components you are > trying to stabilize should be closely thermally coupled to > the sensor. Because there are more than one component to > stabilize, not just the crystal, it is a difficult task. > > The issue is not just static regulation, but also regulation > in the presence of a temperature gradient. We understand that > if you change the ambient temperature by X, the oven will > react to keep the sensor's temperature constant, but the > distribution of that heat flux will change dynamically > (particularly it will be affected by the masses that are in > the way, between the source of heat and the components that > are to be stabilized), and even if you have perfect static > regulation, in most cases there will be a non-zero dynamic > response (a transient). > > The designers of the HP E1938 (which never went to full > production) went through pains to try and keep the gradient > evenly distributed precisely for that reason. My guess is > that it you take the cover out from the E1938, you will find > a perfectly symmetrical layout around the center, where the > thermistor is located. > > A large massive cover over the OCXO will go a long way to > reduce the transient and allow the oven controller loop to > react smoothly, but as long as the time constant is less than > the period of variation (a day?), the peak to peak frequency > variations will not change much, but they should be more > predictable and may be compensated by a feed-forward > mechanism, or a Kalman filter. > > Didier > > > -----Original Message----- > > From: [EMAIL PROTECTED] > > [mailto:[EMAIL PROTECTED] On Behalf Of Magnus Danielson > > Sent: Saturday, July 12, 2008 7:50 AM > > To: [email protected] > > Subject: Re: [time-nuts] Double ovened 10811-60158 on ebay > > > > From: "Ulrich Bangert" <[EMAIL PROTECTED]> > > Subject: Re: [time-nuts] Double ovened 10811-60158 on ebay > > Date: Sat, 12 Jul 2008 13:09:55 +0200 > > Message-ID: <[EMAIL PROTECTED]> > > > > Ulrich, > > > > > > I keep wondering if not a passive oven (metal box, > > insulation, metal > > > > box) would be sufficient. Worst case temperature change > > rates would > > > > be significantly reduced such that the oven loop can track it > > > > better. The remaining temperature shift will be less. > > Basically acts > > > > like a lowpass filter. > > > > > > I have been thinking pretty much the same and therefore I > > built me an > > > "outer" box for my 10811 that has abt. 2-3 cm of free air > > between the > > > outer surface of the 10811 and the inner surface of the > box to make > > > the 10811's temperature regulation "look" into the same "thermal > > > leakage impedance" that the designers may have in mind when they > > > decided for the controller's parameters. > > > > > > Then comes 2 cm of massive aluminium which's heat capacity - in > > > conjunction with the surrounding air's thermal resistance - is > > > expected to give the desired "lowpass filter" effect. > > > > > > Due to the nature of my disciplining system I have been able to > > > measure the overall effect of this shielding quite precise: The > > > lowpass effect is well defined by > > > > > > a) an increase of "phase delay" between temperature > changes outside > > > and changes of the oscillator's frequency that is now in > > the order of > > > some hours. > > > > Which does not as a big supprice but rather confirms the expected. > > > > > b) the frequency changes (as far as they seem to be related to > > > temperature) appear to be "smoother" and more predictable > > > > Which also matches expected and other measurements done. > > > > > Nevertheless I have been a bit disappointed that the > > overall frequency > > > changes along the diurnal changes of the surrounding > > temperature in my > > > flat have been pretty much the same in terms of amplitude > with the > > > above mentioned phase delay. > > > > Your time-constant for the lowpass filter does not do much to the > > diurnal frequency. The temperature changes still go > through. The point > > with the passive oven is that it does smooth things out, and could > > potentially make the oven design inside it an easier task. If > > temperature changes is the main problem then it may be all that is > > needed to get the extra margin. It is certainly not a perfect > > solution, but an interesting exercise which may be of help for some > > cases. > > > > > I have then started to compute what thermal time constant may be > > > expected from this arrangement. This is not trivial stuff > > because in > > > contrast to the aluminium's thermal properties their > > counterparts for > > > the surrounding air are not well defined. As far as I > > remember, a BEST > > > case assumption gave a result of 2-3 hours that seems to > match the > > > empirical results. If you are out for time constants that > > are able to > > > smooth diurnal changes I guess you will have to throw in > MUCH more > > > material. > > > > You would also like to build multiple stages to form a > higher degree > > filter. > > The outermost box is really there to create the chamber around the > > isolating air and in there your heavy mass (big > > cap) is gets heated (charged) and chilled > > (discharged) through the air and black-body radiation. > > > > Having the heavy mass fairly directly exposed to the > surrounding air > > will provide better conductivity through drag... > > > > Oh, humidity will change the heat conductivity of the air > noticeably. > > > > Cheers, > > Magnus > > > > _______________________________________________ > > 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. > > > _______________________________________________ > 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. > _______________________________________________ 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.
