Hi Neven, I can also confirm that you should see a decrease in amplitude in peak mode for SS clocking. I use my simple handheld Thurlby Thander PSA2701T peak-reading spectrum analyzer to demo this during my EMC seminars. _______________________ Kenneth Wyatt Wyatt Technical Services LLC Woodland Park, CO Email Me! | Web Site | Blog Subscribe to Newsletter Connect with me on LinkedIn
On Feb 10, 2012, at 12:16 PM, Neven Pischl wrote: > I would like to thank sincerely to all who responded, I appreciate it. I am > not going to react to any discussions on whether it is cheating or not :), it > was not anywhere in my mind when I posted the question and I hope this topic > does not degrade :). > > But, I'd like to summarize a little: > > 1. I do care about the Pk measurements, not only about QP and Avg, because > that is in the specs I am dealing with (some automotive emission requirements) > > 2. I found out, as I suspected and was confirmed in some replies, that if I > change the modulating frequency up to over 20 kHz, then I see reduction with > 100/120kHz RBW also, not only with 1kHz RBW. Hence, there is an effect of the > modulating frequency and the RBW combination on the measured Pk results. > > 3. SSC by using frequency modulation of the clock actually does reduce the > peak value. I have seen some replies saying it does not, and over many years > I have come across people who said the same. However, if you look in any > textbook on FM, you can see that - in the frequency domain - FM causes the > power of the carrier to be distributed into the side-bands, with the total > power the same with or without the modulation. Therefore, because the power > power stays the same, it must be that each of the components in the spectrum, > i.e. the carrier and the two side-lobes, must have a lower amplitude than the > unmodulated carrier. Please, do not confuse the individual amplitudes of > individual spectral components with the amplitude of the signal in the > time-domain, which indeed stays the same. > > In case of a digital pulses, the "carrier" is the fundamental as well as each > of the harmonics. > > Because of the above, and if you look with the infinitely small RBW (i.e. do > the math), SSC does indeed reduce the peak value of each harmonic (and > fundamental). BTW, the "speed" or modulating freqeuncy does not have a > bearing on the level of reduction of the peak values, in such an ideal case. > Only the modulatioin index (similar to "depth") is important, as it defines > how much power of the carrier is put into the side-lobes. > > The issue I was facing was that under the test conditions of the EMC > specification I have to use a specified "wide" RBW. Under that condition, the > modulating frequency is important too - as I found out. > > > Best regards to all, > > Neven > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > <emc-p...@ieee.org> > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used > formats), large files, etc. > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > For help, send mail to the list administrators: > Scott Douglas <emcp...@radiusnorth.net> > Mike Cantwell <mcantw...@ieee.org> > For policy questions, send mail to: > Jim Bacher <j.bac...@ieee.org> > David Heald <dhe...@gmail.com> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
