Hi One more nit to pick...
ADEV looks at frequency differences. You would take the standard deviation of the delta frequencies (F0-F1 etc) rather than the standard deviation of the frequency (F0,F1...) readings. Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Jim Lux Sent: Monday, August 15, 2011 9:31 AM To: [email protected] Subject: Re: [time-nuts] frequency stabilty question On 8/14/11 8:10 PM, Paul Cianciolo wrote: > Folks, > > I amtrying to understand some of the terms used here quite often. > I quoted this from Wikipedia > > An Allan deviation of 1.3×10-9 at observation time 1 s (i.e. t = 1 s) should be interpreted as there being an instability in frequency between two observations a second apart with a relative root mean square(RMS) value of 1.3×10-9. > > Does this mean the observations made were at the very begining and the very end of the 1 second time. > If so what value about all the values in between? What happens if the oscillator deviated far worse than this during the interrim. > A measurement at tau=1 second doesn't say anything about what happened at shorter intervals. > > Or does the measurement consist of making measurements every cycle during that 1 second and then entering all those values into a formula that accounts for them all?? > Nope.. It's if you measured the frequency (instantaneously) at one second intervals, and calculated the standard deviation, that would be the ADEV for tau=1 second. measure at 10 second intervals and you get ADEV(Tau=10sec), etc. That's why you typically see an ADEV as a series of performances at different taus. You can also fill in the gaps in the curve, to a certain extent, because physical oscillators have constraints on what the ADEV can do (i.e. you're not likely to see 1E-9 at tau=1 second, 1E-5 at tau=2 seconds, 1E-10 at tau=3 seconds) In fact, if you do the ADEV measurement and it's NOT a nice curve and has spikes and weirdnesses, that starts to tell you have either a measurement system problem or a problem with your frequency standard. (sort of like spurs in a phase noise plot from 120Hz line interference, or reference clock leakage) As an example of measurement system problems, it's pretty common to see a "hump" in ADEV around 500-1500 seconds (around 15-20 minutes) because of temperature effects on the test equipment or unit under test as the airconditioning/heating cycles on and off. > Maybe a very basic tutorial on this topic would help but I cant find one > > > Signed very confused, > Thank You > > PauLC > W1VLF > _______________________________________________ > 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. _______________________________________________ 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.
