[time-nuts] GPSDO & Crystal Aging

Richard (Rick) Karlquist richard at karlquist.com
Fri Apr 11 11:09:44 EDT 2014


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-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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>>
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