I worked for the HP Santa Clara Division during
the Smart Clock days and knew all the players.
In terms of holdover, the report cited mentions
temperature compensation and "learning" aging.
The temperature compensation was simply a crutch
for the 10811 to fix its tempco problems. The
E1938A had much better tempco and eliminated the
need for this crutch. As for the concept of
learning aging is concerned, there was definitely
no "secret sauce" I ever heard about in all the
Smart Clock powerpoints I sat through. They
simply measured linear aging and possibly its
derivative and hoped that past performance would
predict future results. It did to some extent,
but how well it worked depended on the particular
crystal. A misbehaving crystal could not be
fixed by any cleverness in the algorithm. Attempts
were made to screen crystals to get predictable
ones, and this was someone successful by getting
rid of bad actors. Still, there was no way to
guarantee that a crystal in the future would never
have a jump or sudden change in aging. What was
really needed was an ensemble of oscillators, but
that was not economically competitive with rubidium.
Rick
On 4/11/2014 3:06 AM, Ulrich Bangert wrote:
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-boun...@febo.com
[mailto:time-nuts-boun...@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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