All, Both loosely coupled and tightly coupled differential pair designs work as all authors have previously stated. The following is a list of some of the general differences between the two types of edge-coupled differential pairs: Loosely coupled better impedance control less sensitivity to manufacturing variations lower track density lower even mode return loss lower insertion loss less impedance variation at 2mm connector transitions less impedance variation at BGA escapes transitions less impedance variation at via transitions Tightly coupled worse impedance control greater sensitivity to manufacturing variations higher track density higher even mode return loss higher insertion loss greater impedance variation at 2mm connector transitions greater impedance variation at BGA escape transitions greater impedance variation at via transitions As with all things in interconnect engineering, there are tradeoffs. At 3.125 Gbps and above, it's the impedance mismatches that get you into trouble, especially near connectors, vias and device escapes. It is extremely easy to create unwanted resonances which will cause unexpected losses at the strangest frequences, some easily within the frequency spectrum of data streams. For high speed circuits where trace density is not an issue, I always choose to use loosely coupled differential pair conductors, where impedance control is much more precise, and resonances caused by impedance mismatch can more easily be reduced. regards, scott -- Scott McMorrow Principal Engineer SiQual Interconnect Engineering 18735 SW Boones Ferry Road Tualatin, OR 97062-3090 (503) 885-1231 http://www.siqual.com ------------------------------------------------------------------ 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