[time-nuts] Crystal aging

Ulrich Bangert df6jb at ulrich-bangert.de
Thu Sep 3 15:22:37 UTC 2009


Poul,

> This argument is utttrly bogus: adaptive time-constant PLL's 
> are at least 30 years old, and rather trivial to implement...

it is a complete mystery to me how you manage to critisize ME for something
that TRIMBLE has not implemented!? And exactly because they did not
implement it my argument is anything else than "bogus". 

> In NTPns I start out with a timeconstant of about 4 seconds 
> in order to get rapid capture, depending on your timebase, it 
> will increase to several hours (OCXO) or days (Rb).
> 
> The only "hard" thing about adaptive PLL's is knowing when 
> you have torqued the timeconstant to high, but monitoring 
> frequency of zero-crossing of the residuals (a cheap 
> approximation to a periodogram) solves that easily.

Maybe! But for the sake of clarity: I have been asking how HP/Symmetricom
manage to handle the regulation loop appearantly WITHOUT time constant
adaption/switching and with a fixed time constant that seems to be much too
high to start with for a cold OCXO.  

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, 3. September 2009 13:58
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Crystal aging
> 
> 
> In message <E28A3329130C445DB0A3FA4C3ED0958B at athlon>, "Ulrich 
> Bangert" writes:
> 
> >The true thrill is however one step more subtle: It first 
> considers the 
> >question WHY trimble choose 100 s as the default loop time constant. 
> >Well, this one is easy to answer: Just set the time constant of a 
> >"cold" TBOLT to 1000 and watch the TIC value flying to the 
> moon. Expect 
> >days before the loop locks. Trimble NEEDS to set the deafult time 
> >constant that low in order to make the loop of a cold TBOLT 
> lock within a reasonable time.
> 
> This argument is utttrly bogus: adaptive time-constant PLL's 
> are at least 30 years old, and rather trivial to implement...
> 
> In NTPns I start out with a timeconstant of about 4 seconds 
> in order to get rapid capture, depending on your timebase, it 
> will increase to several hours (OCXO) or days (Rb).
> 
> The only "hard" thing about adaptive PLL's is knowing when 
> you have torqued the timeconstant to high, but monitoring 
> frequency of zero-crossing of the residuals (a cheap 
> approximation to a periodogram) solves that easily.
> 
> -- 
> 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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