[time-nuts] GPSDO & Crystal Aging
Tom Van Baak
tvb at LeapSecond.com
Thu Apr 10 20:05:41 UTC 2014
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.
> AFAICR the HP GPSDOs included the idea of measuring the aging rate of the crystal and applying that correction during
> 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.
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