Hi > On Aug 18, 2016, at 12:59 AM, Charles Steinmetz <[email protected]> wrote: > > Bob wrote: > >> The point is still looking at the noise characteristics of the oscillator >> and the reference. >> It is best done in the frequency domain as phase noise. We substitute ADEV, >> but that >> is not an ideal proxy. > > Phase noise and xDEV measure the same thing -- the stability of an oscillator > at different time scales. They just express the result differently. Phase > noise expresses it as PM in the frequency domain, and xDEV expresses it as > "parts per" in the time domain. (Yes, this is a somewhat simplified view of > it, but it captures the essential point without undue complexity.)
ADEV is quite poor at frequency discrimination. That problem really nailed HP back in the 1970's. You will have a much easier time doing it with a FFT / frequency domain data set. > > Conventionally, we switch from using PN to using xDEV at a time scale > (reciprocal frequency scale) of around 1 second, but there is no mathematical > reason why they both cannot be extended indefinitely in either direction. > The convention arose largely because the equipment and techniques we use[ed] > to quantify them have traditionally been different at time scales (reciprocal > frequency scales) greater than and less than about one second. Now that we > are in the era of "digitize everything, and let Laplace sort it out," we > needn't view it as the rigid convention it once was. > >> Either way you want the loop to cross over from one to the other >> somewhere in the vicinity of the “equal noise” point if it exists. If there >> is no equal noise >> point, that makes you wonder a bit about why you are locking one to the other > > Not really, if one has lower noise at all time (frequency) scales, just lock > to that one at all scales. (It may call into question why you're fiddling > with two oscillators, rather than just using the output of the quiet one, if > they are both at the same frequency -- but there are a number of reasons one > might want to do that.) Thus no equal noise and the obvious question.... Bob > > Best regards, > > Charles > > > _______________________________________________ > 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.
