The harmonics limit the perfomance if you want to find the frequency of a given, sampled sinewave with linear methods. Thats at least my finding when I built a device to measure grid frequency fast and with high accuracy. I had to use high-Q digital filters for the fundamental. Their slow transient response limited the speed of new frequency measurements.
Cheers Detlef DD4WV "time-nuts" <[email protected]> schrieb am 23.11.2017 16:34:39: > Von: Tim Shoppa <[email protected]> > An: Discussion of precise time and frequency measurement <[email protected]> > Datum: 23.11.2017 16:35 > Betreff: Re: [time-nuts] Allan variance by sine-wave fitting > Gesendet von: "time-nuts" <[email protected]> > > I wonder how much a fitting approach is affected by distortion (especially > harmonic content) in the waveform. > > Of course we can always filter the waveform to make it more sinusoidal but > then we are adding L's and C's and their tempcos to the measurement for > sure destroying any femtosecond claims. > > Tim N3QE > > On Wed, Nov 22, 2017 at 5:57 PM, Ralph Devoe <[email protected]> wrote: > > > Hi, > > The fitting routine only takes up 40 uS of the 1 sec interval > > between measurements, as shown in Fig. 1 of the paper. This is less than > > 10(-4) of the measurement interval. It just determines the phase difference > > at the start of every second. I don't think the filtering effect is very > > large in this case. > > The interesting thing is that good results are achievable with such > > a short fitting interval. One way to think of it is to treat the fitting > > routine as a statistically optimized averaging process. Fitting 40 uS, that > > is 4096 points at 10 ns/point, should reduce the noise by a factor of 64 > > (roughly). The single shot timing resolution of the ADC is about 10 pS (see > > Fig. 4), so dividing this by 64 brings you down into the 100's of fs range, > > which is what you see. > > > > Ralph > > _______________________________________________ > > 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.
