[time-nuts] GPSDO & Crystal Aging

Ulrich Bangert df6jb at ulrich-bangert.de
Fri Apr 11 06:06:31 EDT 2014


Hi Brooke,

> HP had some way around SA that improved the timekeeping.

HP called it the "Smartclock Algorithm" and you can find some very basic
information about it here:

http://www.hpl.hp.com/hpjournal/96dec/dec96a9.pdf

I have been trying months to find a reference on how it REALLY works but it
seems that this is one of the better kept secrets of HP.

Best regards

Ulrich

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Brooke Clarke
> Gesendet: Donnerstag, 10. April 2014 22:56
> An: Tom Van Baak; Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] GPSDO & Crystal Aging
> 
> 
> Hi Tom:
> 
> That makes sense because the GPS was just coming on line and 
> not anywhere near a full compliment of satellites and SA 
> was on.
> HP had some way around SA that improved the timekeeping.
> Has that ever been disclosed?
> 
> Have Fun,
> 
> Brooke Clarke
> http://www.PRC68.com 
> http://www.end2partygovernment.com/2012Issues.html
> 
> Tom Van Baak wrote:
> > Hi Brooke,
> >
> > True, except that in most cases the long-term frequency 
> drift rate is 
> > so tiny compared to all the short- and mid-term instability 
> that it is 
> > not worth worrying about. In other words, I agree it is 
> modeled as a 
> > "linear ramp", but the ramp, even at huge timescales, is so 
> close to 
> > flat, what's the point?
> >
> > Look at the output of a typical OCXO. Short-term the 
> frequency varies 
> > by tens or hundreds of ps/s; that's parts in 10^11 or 10^10. By 
> > contrast, you have wait an entire day or week before you get that 
> > level of frequency error due to drift.
> >
> > When you're in a rowboat outside SF bay, it's the 3 m waves 
> every 5 to 
> > 10 seconds that you need to steer against, not the 3 m tides that 
> > occur gradually over 12 hours.
> >
> > Can someone show me a counter-example? Why is it better to include 
> > aging rate into the PID. What quantitative improvement in 
> performance 
> > does this actually represent? I don't disbelieve it, I just 
> have never 
> > seen the numbers.
> >
> > One case where knowing the aging rate is important is during 
> > multi-hour or multi-day holdover. Perhaps that's why HP 
> included the 
> > 128-hour circular record of frequency/aging into their firmware.
> >
> > /tvb
> >
> >> Hi:
> >>
> >> AFAICR the HP GPSDOs included the idea of measuring the 
> aging rate of 
> >> the crystal and applying that correction during holdover. This was 
> >> also mentioned by Brooks Shera in relation to his GSPDO 
> (there was a 
> >> plot), but I don't think it was part of the firmware?
> >>
> >> So rather than just locking the control voltage to the last used 
> >> value it would be much better to add a linear ramp. 
> >> <http://www.rt66.com/%7Eshera/>
> >
> >
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