Hi While the units are more properly femto seconds/ second you do indeed see 1s AVAR plots labeled in fS.
Bob On Jun 20, 2010, at 6:25 PM, Magnus Danielson wrote: > On 06/20/2010 11:53 PM, jimlux wrote: >> Robert Benward wrote: >>> Bob >>> Boy, you guys are really making me read a lot. I'm digesting Wiki >>> right now. >>> >>> I see tau, but does identifying a tau of 1E-14 allow you to say you >>> are locked to 10fs? The smallest tau I've seen in my E1938 collection >>> is 1E-1. >>> >>> Bob >>> >> >> tau is the time over which the measurement is made, typically 1 second >> or greater. >> >> loosely speaking, the 1e-14 is the average fractional deviation of >> frequency over that time period. > > It is a RMS type (much like statisticians standard deviation) of frequency > stability over the "observation interval" of tau (little greek letter looking > similar but not quite like a little t, which is the real reason for using > it). Since it is a RMS type of measure, it indicates the effective power of > noise, but not what the actual deviation in frequency will be, it's just a > statistical measure. You may form a confidence interval such as that for 99,7 > % or something which forms a scale-factor, quite similar to the use of the > error function for the Gaussian distribution. > > An Allan deviation measure of 1E-14 is however not quite the same as 10 fs. > Besides the units being wrong (Allan deviation is a relative and unit-less > measure, essentially Hz/Hz) the Allan variance (and hence the Allan > deviation) is a frequency stability measure, indicating the stability of > normalized frequency rather than stability of normalized phase. The time > deviation represents the stability of phase over some observation time. > Assuming the nominal frequency and linear effects removed, then this would > indicate the time error noise of the phase, here use of seconds could be > used, but it would be to stretch things a bit. > > The time and frequency world has it's own qualities of noise... > > Cheers, > Magnus > > _______________________________________________ > 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. > _______________________________________________ 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.
