Hi, Consider, first, the ideal case. Considering edge coupled differential traces over a plane(I suppose that one could generalize to any kind of differential pair over a plane). If the legs of the pair, in the ideal case, were identical in length, width, etc, I would expect that there would be no differential to common mode conversion and there would be no return current through the ground plane. If there is no return current in the ground plane then I would expect that if the ground plane was "choked" with ferrites (these are ideal ferrites that take up no physical space) at some point in between the two ends of the differential pair, then the odd mode impedance would not be affected. I would expect that the even mode impedance would increase as a result of the ferrites. Now consider the real world. In a PCB, in order to "choke" the ground plane underneath the differential pair I would first have to physically cut the plane, bring vias to the surface and then use a ferrite chip to connect the two sections of plane. My question is, does that physical cut cause an increase in the odd mode impedance (and, hence, a discontinuity) of the differential pair, at the point of the cut in the plane? I would expect the answer to be yes. Thank You Tom tom_cip_11551@xxxxxxxxxxx ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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