As I mentioned earlier, the waveform from a switching power supply has two distinct components, due to the fast rise-time driving current into ground (common mode) and the pulse itself which is differential mode. Separating modes allows you to time window properly to really resolve the waveform to be FFT'd. In my experience, that makes a world of difference. To specify further, I had maybe 32 k of memory. If you have infinite memory, you can FFT a 10 us pulse with a sub-microsecond rise-time, but with limited memory, you get better results doing a separate FFT on the rise-time and then the pulse.
> From: John Woodgate <j...@jmwa.demon.co.uk> > Reply-To: John Woodgate <j...@jmwa.demon.co.uk> > Date: Sun, 27 Jul 2003 07:46:29 +0100 > To: emc-p...@majordomo.ieee.org > Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser > > > I read in !emc-pstc that Charles Grasso <cgrassospri...@earthlink.net> > wrote (in <ekeeipjkkmpklafoobmcoelhcfaa.cgrassospri...@earthlink.net>) > about 'Q on Correlation of Votage ripple with a Spectrum Analyser' on > Sat, 26 Jul 2003: > >> Has anyone tried correlating the voltage ripple as seen on a scope with >> the amplitudes measured on a Spectrum Analyser? > > More information, please. What voltage is it that has ripple on it? Do > you mean the ripple voltage across a rectifier filter capacitor? >> >> I tried doing that the other day with ..umm. minimal success. I think >> that due to the comples convoltions that would have to occur when >> FFT'ing an irregular voltage shape. > > It's really no more difficult to FFT one waveform than another. The FFT > doesn't 'know' whether the waveform looks regular or irregular to a > human. > > If it is power supply ripple that you are considering, the waveform is > approximately a sawtooth, but the short rising branch is a small section > of the crest of a sine wave. If the ripple voltage is very small > compared with the d.c. voltage and the ESR of the filter capacitor is > very low, the spectrum of the ripple is often quite close to that of a > sawtooth. > -- > Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk > Interested in professional sound reinforcement and distribution? Then go to > http://www.isce.org.uk > PLEASE do NOT copy news posts to me by E-MAIL! > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: emc_p...@symbol.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > Archive is being moved, we will announce when it is back on-line. > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
