The (modal) field distribution on the transmission line depends on the frequency for various reasons: - material dispersion (the dielectric constant of the substrate is slightly frequency dependent) - structures with inhomogeneous dielectrics (microstrip as opposed to stripline) are dispersive even if the materials are not ? at higher frequencies the field tends concentrate more in the dielectric with higher dielectric constant - skin and proximity effects in the trace metal are also frequency dependent This translates into L and C (and G and R) frequency dependent ie characteristic impedance and propagation constant also frequency dependent Peter Pang Ning <pangning2000@xxxxxxxxx> wrote:Dear all, I have a confusion about the PCB transmission line impedance. As I knew before, the transmission line impedance depends on the square root of L/C. That is to say, signal switching frequency will not impact transimission line impedance. But just now one famous PCB manufacturer informed me that they can't control differential pair impedance to 100ohm at the 1.25GHz frequency. I felt confused about the frequency will impact transmission line impedance. Does anybody can give me some idea about this question? Thanks a lot! Best Regards, Peter Pang ------------------------------------------------------------------ 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: http://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: http://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!? Yahoo! Mail - Find what you need with new enhanced search. Learn more. ------------------------------------------------------------------ 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: http://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: http://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