> On the subject of Brooks Shera's design, the one thing that troubles me is the > use of a 24 MHz oscillator to count the width of the 1PPS signal. > This yields a precision of 4.16e-8, but does it really?
No, with averaging it's much better than that. > This oscillator is uncontrolled and any drift would exist as noise that would > have to be filtered (He uses a software low pass filter). No, when an oscillator is used as a timebase for what is essentially a short period time interval counter the XO drift rate does not affect the result like you think. Suppose you use a cheap XO with a huge drift rate of 100 ppm per year or even 1 ppm per day to make TI measurements between the OCXO and GPS. So an average measurement that is, say 12.34 ns today, will be off by 1 ppm tomorrow: it will be 12.34001 ns instead. Do you see now why it doesn't matter how bad the XO is? Secondly, someone can double check me here -- but it seems to me that any GPSDO that uses a built-in TIC to monitor the deviation between the GPS 1PPS and the OCXO 1PPS is a closed loop system and so the actual accuracy of the TIC timebase has no effect on the function of the GPSDO. I mean, the 24 MHz clock could drift down to 20 MHz or up to 30 MHz and the GPSDO would still work fine (hey, maybe even better). /tvb _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts