[time-nuts] Double ovened 10811-60158 on ebay

Didier Juges didier at cox.net
Sat Jul 12 10:01:14 EDT 2008


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: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] 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: time-nuts-bounces at febo.com
> > [mailto:time-nuts-bounces at febo.com] On Behalf Of Magnus Danielson
> > Sent: Saturday, July 12, 2008 7:50 AM
> > To: time-nuts at febo.com
> > Subject: Re: [time-nuts] Double ovened 10811-60158 on ebay
> > 
> > From: "Ulrich Bangert" <df6jb at ulrich-bangert.de>
> > Subject: Re: [time-nuts] Double ovened 10811-60158 on ebay
> > Date: Sat, 12 Jul 2008 13:09:55 +0200
> > Message-ID: <769FE70BC9AD430CA6506EEEC90E3B8B at athlon>
> > 
> > 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
> > 
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> 
> 
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