How critical the characteristic impedance of via transition is? It depends on the bit rate, channel insertion loss and the number of vias on each channel.=20 BR Tony -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Harry Selfridge Sent: Wednesday, January 09, 2008 6:50 PM To: 'SI LIST' Subject: [SI-LIST] Re: 50 Ohm Via? There was an article written about controlled impedance vias several=20 years ago by Thomas Neu of Texas Instruments. I haven't seen any=20 followup articles by anyone on the subject since. You can read Neu's=20 article online at: http://www.edn.com/index.asp?layout=3Darticle&articleid=3DCA324403 . Others may have experienced different results, but I've never found=20 controlled impedance vias to be necessary or useful. The distances=20 involved in a via are so short that any pretense of matching=20 impedance is negligible compared with other variations that you might=20 encounter over the full length of a signal path. One board we built=20 for a customer provided two signal paths, one with Neu's controlled=20 impedance vias, and duplicates without. Testing of the loaded board=20 showed no appreciable difference in performance, and the loss of=20 board space to the structure necessary to achieve the controlled=20 impedance vias was considerable. Regards - Harry At 05:51 PM 1/9/2008, you wrote: >Is there such a thing as a design methodology for designing a PCB via with >50 ohm impedance, or does it have to be done iteratively using a 3D field >solver? >Are controlled impedance vias necessary, worthwhile or helpful for >multi-gigabit serial links running at 1 to 5 Gbps? > > > >Thanks - Joel ------------------------------------------------------------------ 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: =20 //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 =20 ------------------------------------------------------------------ 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 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