I am working with a layout that uses diff pairs routed as stripline on internal layers and micro strip on outer layers. These include PCI Express, USB, LVDS, Ethernet. There are guidelines I have read that recommend spacing between pairs and between other signals should be at least 3x dielectric height for stripline and 4x dielectric height for microstrip. I am using 13.5 mils dielectric height for the internal layers which means I need spacing of 40 mils. If I want to increase my routing density (decrease spacing) then I would need to decrease the dielectric height. In order to do this I would have to increase the inter pair spacing to maintain 100 ohms impedance which would reduce the inter pair coupling and increase the coupling to the reference plane. This means more current would flow on the plane. My understanding is that this plane current flows in a circular loop on the plane underneath the diff pair traces essentially cancelling itself out to some degree. My question is there any issue (EMI or other) with forcing more of the return current to flow on the planes? If density was not an issue would it be desirable to make the dielectric height as large as possible to reduce reference plane current? 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: //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