Concerning Zo relation with frequency: Once again, depends where you live. On boards, typically the frequency-dependent impedance change starts leveling out at much lower frequencies. Essentially you are approaching the sqrt(L/C) impedance, because your omega*L overwhelms your R. *Usually* by 1MHz your Zo curve is flat. But if you are modeling chip lines, your R value for the line might be comparable (or greater) than omega*L up to a couple GHz or so. Then you cannot ignore this frequency dependent behavior. I have seen a typical line on chip go from about Zo=100ohms @100MHz, to Zo=63ohms @1GHz, to Zo=55ohms @10GHz. Measurement, simulation, theory, literature, and gut feel all back this up. So the bottom line (as it always is in the world of interconnect modeling) is it depends on how high you go in frequency, the dimensions of line you are using, and if you are designing in a narrow band or a wide band. HOWEVER, as Jon pointed out, you can often see greater variations due to coupling from nearby traces. Plus you have to remember that impedance control is an issue too - usually +/- 10% is as good as it gets for run-of-the-mill PCBs out there (but money talks). To get a feel for the numbers I got above, you can use a 2D field solver that handles the frequency dependent behavior of R and L (ansoft spicelink or some other flavor). Or you can dig up equations and plug them into a matlab or mathcad. Calculate your R and L and C (usually G is non-existant or insignificant...) and crunch away. salud, Andy Byers -----Original Message----- From: Jon Powell [mailto:jonpowell@xxxxxxxxxxxx] Sent: Wednesday, September 17, 2003 9:09 AM To: kbagga31@xxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: impedance relation with frequency... Karen, It is my feeling that the frequency related impedance changes on a signal will be second order considerations compared to the impedance changes caused by crosstalk from neighboring wires. These effects can be shown with most good SI engines. Intel has often recommended (for instance) calculating the effective impedance when the coupled wires on either side of the target wire switch simultaneously with the target wire in both the same direction (all going high and low) and opposite (target going high and low and coupled going low and high). hope this helps (and if I am wrong, I am sure someone will scream at me so wait a couple of minutes). regards, jon -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of karan bagga Sent: Wednesday, September 17, 2003 2:24 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] impedance relation with frequency... Hi From the telegraphic equations on Transmision lines it seems the impedance of the Trace varies with frequency. In my design specifications it is specified that my trace should be on (25 +/- 10%) Ohms. How will I do it ? How will I do these kind of analysis? The frequency of the signal is high and also the rise time is significantly low. Will FFT be of some help here ? Regards Karan. --------------------------------- Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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