Not really sure what you have in mind by *ideal* trapezoidal shape? Whatever it is, if we look from the frequency domain view: one can take a Laplace / Fourier transform (depending on periodic nature of the waveform), and result will be a summation of sinusoidal waveforms with various Amplitudes. So there is always AC component to the signal. --- Doug Brooks <doug@xxxxxxxxxx> wrote: > Mmmmmmm, > Well.............. > Draw a picture of an *ideal* trapezoidal repetitive > waveform and then think > about what you said. > During transition time, there is a high di/dt > component. (hence a high > coupling component to the plane). > > During the part of the waveform where the signal has > *stabilized* the > instantaneous di/dt is very low or zero (hence a > lower coupling component > to the plane). > > Nevertheless that part of the signal changes > polarity twice a cycle. If you > want to think there is no AC component there, that > is your prerogative. I > happen to think there is. > > Doug Brooks, PhD > > > > > > At 11:16 AM 4/20/2006, Scott McMorrow wrote: > >Doug > > > >I beg to differ. If the signal has "stabilized" > there is therefore no AC > >component. If there is no AC component, there is > nothing to radiate. > > > >Scott > > > > > >Scott McMorrow > > > > > >Doug Brooks wrote: > >> > >>In my humble opinion, and not counting common mode > currents: > >> > >>During the signal rise and fall times, the return > current tends to flow on > >>the reference plane, just as signals on > single-ended traces do. > >> > >>During the time that the signal is "stabilized," > there is no coupled signal > >>on the plane and the loop is around from one trace > of the differential pair > >>to the other. > >> > >>It is during this latter phase of the signal that > loop area (as in EMI) > >>might be an issue. During my signal integrity > seminars I show some > >>animations that illustrate this pretty clearly. > >> > >>Doug Brooks > >> > > > >____________________________________________________________________________- > >Check out UltraCAD's differential impedance and > skin effect calculators at > >http://www.ultracad.com > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in > the Subject field > > or to administer your membership from a web page, go > to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the > Subject field > > List FAQ wiki page is located at: > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are > viewable at: > http://www.qsl.net/wb6tpu > > > __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu