The .35, and actually the whole equation comes from filter theory of an RC circuit. Here is the short form of the derivation... If you have a source u(t), a resistor (R) and capacitor (C) in series the voltage at the junction between the R and C is... Vc = 1 - e^(-t / RC) t = -RC * ln (1 - Vc) Since BW = 1 / 2*pi*RC t = (-1 / 2 *p*BW) * ln (1 - Vc) Using a risetime from 10 -90% : Tr = t(0.9) - t (0.1) = -(1/ 2*pi*BW) * [ln (0.1) - ln (0.9)] Therefore Tr = 0.349699 or Tr = .35 / BW Regards, -----Original Message----- From: Keskinen, Kai [mailto:KaiKeskinen@xxxxxxxxxxxxx] Sent: Tuesday, May 20, 2003 12:24 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Interconnect Simulations-V2 If you work out the amplitude of the harmonics of an infinitely repeating trapesoidal waveform with equal rising and falling edges, you find that the harmonics on a log-log plot fall at 20dB per decade until you reach 1/(pi*Tr) ~ 0.318/Tr . After that knee frequency, they fall at 40dB per decade. Most of the significant harmonics are present to the knee frequency. Clayton Paul's book on EMC has both measurements and the derivation for those who last did a fourier analysis in undergrad. Does anyone know where the 0.35 number can be derived from? Cheers, Kai Keskinen Signal Integrity Advisor Solectron Technical Center 21 Richardson Side Road, Kanata, ON K2K 2C1 <mailto:kaikeskinen@xxxxxxxxxxxxx> <http://www.solectron.com/> "This message may contain information proprietary to Solectron so any unauthorized disclosure, copying or distribution of its contents is strictly prohibited." ------------------------------------------------------------------ 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 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 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