[time-nuts] GPSDO & Crystal Aging

Brooke Clarke brooke at pacific.net
Thu Apr 10 20:55:45 UTC 2014

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

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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