[time-nuts] Thunderbolt stability and ambient temperature

Ulrich Bangert df6jb at ulrich-bangert.de
Thu Jun 11 08:16:05 EDT 2009


> I'm not talking about crystal ovens, I'm talking about 
> timekeeping kit in general: how to filter high frequency 
> temperature fluctuations out, so the PLL does not have to 
> deal with them.

Ok, but in this general case you have to accept the fact, that filtering
high temperature fluctuations is only ONE part of the engineering job. The
second task (which I tried to point at) is: When you do already have a
closed loop temperature regulation system inside what you plan to built
around it, then you need to manage the filtering without mal interaction of
the local temperature controller with your additional filtering stuff. The
art is to make the closed temperature loop inside see only smaller
temperature fluctuations but No changes otherwise. 

If you apply an overall change in temperature insulation to the ambient you
will make the work of the inside temperature controller a lot harder because
it is not prepared for that. In the aluminium box that i wrote about I have
for an experiment replaced the air between the 10811 and the outer box with
a material that has a significant lower temperature conductivity than
standing air. You can easily watch the temperature controller go crazy with
that. For a second experiment I have replaced the air with a material having
lots more of thermal conductivity than air and you can watch the temperature
controller go crazy with that too. You don't need sophisticated temperature
measurement equipment to see the controller go crazy, just watch the
frequency of the oscillator. This will tell you all about it. 

Things are different, if you have an influence on the controller's
regulation parameters (which you do not have with a ready bought device). If
you do have, then your additional provisions for filtering and insulation
can easily be included into the thermal model that the temperature
controller has to handle and will lead to different operation parameters of
the controller. That will of course work and that is why I compared your
suggestions to designing a new oven (for xtal or rubidium or whatever). 

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Poul-Henning Kamp
> Gesendet: Donnerstag, 11. Juni 2009 12:50
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Thunderbolt stability and ambient temperature
> 
> 
> In message <E182724B9FCE4B9F9E914BCC4F374401 at athlon>, "Ulrich 
> Bangert" writes:
> >> For PLL steered devices, you want your device enclosed by a
> >> thermal mass which is again enclosed by a layer of thermal 
> >> isolation.  The goal is to filter/average all rapid (daily ?) 
> >> external temperature influences, only letting through such 
> >> slow variations (seasonal ?) which the PLL can comfortably 
> cope with.
> >
> >My impression with this thread is that two sligthly different things 
> >are being discussed here:
> >
> >Some posters explain how a crystal oven shall be designed as 
> >Poul-Henning does with his words above. Absolute correct, no doubt 
> >about it!
> 
> I'm not talking about crystal ovens, I'm talking about 
> timekeeping kit in general: how to filter high frequency 
> temperature fluctuations out, so the PLL does not have to 
> deal with them.
> 
> The above advice applies to GPSDO's and Rb's and Cs's as well 
> as TCXOs, OCXOs and voltage references.
> 
> Poul-Henning
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by 
> incompetence.
> 
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