Joel, theoretically, if there really is no stimulus in the system, then of course any resonance of the PDN at that frequency is unimportant. In practice though things usually aren't so clear-cut. Remember that if you have e.g. a digital data stream, that contains a virtual continuum of frequency components from DC all the way up to the knee frequency (0.5/Trise) and even beyond. That's virtually guaranteed to excite that resonance and thus create additional power supply noise. So even a a single clock frequency the power supply current profile will have frequency spectrum. Also, unless you spend an extraordinary effort on simulation, modeling and measurement, chances are you can't predict where your resonances are with sufficient accuracy to guarantee they won't lie in a "bad" spot. Add to that manufacturing and component tolerances and you'll soon find it's easier to just do a clean PDN design (impedance well below target over the whole frequency range of interest) rather than count on your luck. Just my 2 cents Wolfgang "Joel Brown" <joel@xxxxxxxxxx> Sent by: si-list-bounce@xxxxxxxxxxxxx 08/20/2009 01:16 PM To <si-list@xxxxxxxxxxxxx> cc Subject [SI-LIST] PDN question Is it really necessary to have a PDN with impedance flat or below a target over a wide range of frequencies? If a resonance occurs is it really a problem if there are no switching currents at that frequency? I am assuming that switching frequencies are a function of the clock frequencies in the system. Thanks - Joel ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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 ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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