Thank you Scott and Steve, But, What is the benefict to use loosely coupled pairs? When I need to use it? when I have common mode terminations? Rgds, On Tue, Nov 9, 2010 at 6:02 PM, steve weir <weirsi@xxxxxxxxxx> wrote: > It's basically telling you to use loosely coupled pairs where each line has > a single ended impedance of 50 Ohms. For zero line to line coupling: Zcm > > Zse /2, and Zdiff = Zse*2. > > Steve. > > > Alvaro alkschbirs wrote: > >> Hi gurus, >> I have a question about differential pairs routing. >> >> The XFP MSA Specifications says: >> >> "XFI differential reference impedance for driver, *transmission line*, and >> receivers are defined as 100 Ohms. XFI common mode reference impedance for >> driver, *transmission lines*, and receivers are specified as 25 Ohms." >> >> I think that it says that my signal need to have both 25ohms commom mode >> impedance and 100ohms differential impedace. In pratical terms, I need to >> make this traces thinner and more spaced. Thinner traces are more critical >> and add too. >> >> Is this requirement true for any type of 100ohms differential impedance >> pair? >> >> If yes, this requeriment will add some problems to make BGA fanouts >> without >> route the pairs separately by other pins fanout vias. >> >> Why I need control common mode impedance? >> How critical is this requirement? >> Is 30 ohms commom mode impedance acceptable? >> >> I will appreciate your help! >> Rgds, >> Álvaro >> >> ------------------------------------------------------------------ >> 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 >> Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> >> >> > > > -- > Steve Weir > IPBLOX, LLC 150 N. Center St. #211 > Reno, NV 89501 www.ipblox.com > > (775) 299-4236 Business > (866) 675-4630 Toll-free > (707) 780-1951 Fax > > > ------------------------------------------------------------------ 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 Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu