*From*: steve weir <weirsi@xxxxxxxxxx>*To*: Vinu Arumugham <vinu@xxxxxxxxx>*Date*: Tue, 09 Nov 2010 13:09:24 -0800

If the Z matrix has essentially zero line to line coupling at the transmitters and receivers, then I am at a loss as to how a channel that does not match that will not suffer some amount of mode conversion. The reality is that a perfect match is unattainable. But the further one deviates, the more degradation will result. Steve. Vinu Arumugham wrote: > I don't think the cited section of the spec. below can be interpreted as > requiring loosely coupled 50 ohm single ended lines. > "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." > > The reference impedance is the port reference impedance used to check > compliance of the driver, receiver or transmission line parameters. > > I don't think the spec. places a direct limit on the actual common mode > impedance of the interconnect. Only differential insertion and return > loss requirements are specified. So, a 90 ohm differential pair with 30 > ohm common mode impedance could be compliant. > > Thanks, > Vinu > > steve weir 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: >>> http://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: >>> http://www.freelists.org/archives/si-list >>> >>> 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: > http://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: > http://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: http://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: http://www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**Follow-Ups**:**[SI-LIST] Re: Differential pairs commom mode impedance***From:*Vinu Arumugham

**References**:**[SI-LIST] Differential pairs commom mode impedance***From:*Alvaro alkschbirs

**[SI-LIST] Re: Differential pairs commom mode impedance***From:*steve weir

**[SI-LIST] Re: Differential pairs commom mode impedance***From:*Vinu Arumugham

- » [SI-LIST] Differential pairs commom mode impedance - Alvaro alkschbirs
- » [SI-LIST] Re: Differential pairs commom mode impedance - Scott McMorrow
- » [SI-LIST] Re: Differential pairs commom mode impedance - steve weir
- » [SI-LIST] Re: Differential pairs commom mode impedance - Alvaro alkschbirs
- » [SI-LIST] Re: Differential pairs commom mode impedance - steve weir
- » [SI-LIST] Re: Differential pairs commom mode impedance - Jory McKinley
- » [SI-LIST] Re: Differential pairs commom mode impedance - Scott McMorrow
- » [SI-LIST] Re: Differential pairs commom mode impedance - Vinu Arumugham
- » [SI-LIST] Re: Differential pairs commom mode impedance - steve weir
- » [SI-LIST] Re: Differential pairs commom mode impedance - Vinu Arumugham
- » [SI-LIST] Re: Differential pairs commom mode impedance - steve weir
- » [SI-LIST] Re: Differential pairs commom mode impedance - Vinu Arumugham