[time-nuts] How do I measure oscillator frequency using 1pps?

Tom Van Baak tvb at leapsecond.com
Sat Jul 9 21:25:33 EDT 2005

> If you look at the Time Interval once per day you will have 9 ns of 
> sigma on the GPS signal which divided by 86,400 seconds is 1E-13.  But 
> the 9 ns number gets divided by SQRT(n) where n is the number of 
> readings you are averaging, so in my case of 1,000 averages the 1 day 
> accuracy is about 3E-15.

The sqrt(N) trick only works when the N samples
are uncorrelated. With your setup you will get
some advantage in averaging but not sqrt(N).

> Another option is to use a computer to read the negative sawtooth 
> correction from the GPS receiver and use that on a second by second 
> basis to correct the 1 PPS TI.

A plain M12+ will give you about 9 ns rms; applying
the sawtooth corrections drops that down to about
3 ns rms. See:



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