Hi The "small angle" assumption also tends to break down when testing phase noise under significant amounts of vibration.
Bob On Aug 31, 2013, at 7:00 PM, John Miles <[email protected]> wrote: > Normally any higher-order sidebands are ignored when dealing with PM. You > can think of it as NBFM with a very low modulation index -- all of the > "intelligence" is in the first sideband. > > Put another way, the modulation index for FM is the peak frequency deviation > divided by the highest frequency present in the modulating signal. In phase > modulation the carrier frequency is normally considered constant. So the > numerator of the fraction you would use to index a table of Bessel functions > is 0, yielding a result of 1. > > It gets more complicated when the PM at a given offset frequency shifts the > carrier by a radian or more. This is where the small-angle assumption for > standard L(f) phase noise measurements becomes invalid. If you are going to > measure the phase noise of a Cs standard at offsets of small fractions of a > Hz, this will eventually be an issue given the steep PN slope close to the > carrier. Is that what you're doing? > > For finer-grained Bessel resolution it make sense to evaluate the function > yourself. I think there's a BESSEL() keyword in Excel, and Matlab or Octave > could certainly do it. > > -- john, KE5FX > Miles Design LLC > > >> -----Original Message----- >> From: [email protected] [mailto:[email protected]] On >> Behalf Of [email protected] >> Sent: Saturday, August 31, 2013 11:52 AM >> To: [email protected] >> Subject: [time-nuts] Phase modulation question >> >> Hi, >> >> I'm making some phase mod index measurements as it relates to Rubidium >> and Cesium standards. >> >> Two questions. >> >> 1. Is a Bessel table for sine wave modulated FM used for PM also? >> >> 2. Where can I find a Bessel table that has fine resolution, the ones I'm >> seeing only have large increments of Mod index. (like .25 .5 1.0 1.5 >> 2.0) I need to measure in between these values. >> >> Thanks, >> >> Corby >> >> _______________________________________________ >> 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.
