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/> _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
