[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
Tom Van Baak
tvb at leapsecond.com
Thu Dec 14 01:47:07 EST 2006
> On the subject of Brooks Shera's design, the one thing that troubles me is
> 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
> 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).
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