Actually, it depends on the ratio of the distance between the lines in the pair and the distance to the ground plane. If the ground plane is much much closer than the spacing between the lines, then return current should flow through the ground plane primarily. Having said that, we have experimented with varying that ratio, and checking how much effect it will have on the measurement of differential impedance. The motivation was to test the effectiveness of the P80318 differential TDR probe that Tektronix makes when it is used without ground leads - differential contact only (with differential contact only, this probe is amazing in its ability to stay in contact with the differential DUT pair despite shaky operator hands, varying angles of attack etc.) The surprising result was that even for fairly sizeable ratios between spacing to ground and spacing between the lines, the impedance measurement have not changed. Having said THAT :-), I still content for very large spacing between the lines, the interaction to the ground plane will dominate. Thanks, =20 Dima Smolyansky Tektronix, Inc. P.O. Box 500, MS 39-111 Beaverton, OR 97077-0001 503-627-7180 503-627-2260 (fax) -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Aleksandr Oysgelt Sent: Thursday, April 20, 2006 9:53 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Question regarding return current in a differential pair In presence of a reference plane most (appr 90%) of return current flows on reference plane. When no reference plane is present (such as twisted pair cable) all return current will flow on other wire in pair. Aleks -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Michael Kotson Sent: Thursday, April 20, 2006 9:47 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Question regarding return current in a differential pair Hello Gurus, I have a question regarding the return current for a high speed differential signal. I've seen a couple of sources that state that all of the signal return current is handled by the other half of the differential pair. It seems more logical to me that there must be some amount of return current (not just common mode) on the reference plane(s) as well. What's the truth? =20 Thanks in advance, Mike =20 Michael Kotson Project Engineer VT Miltope=20 4900 Pearl East Circle Suite 106 Boulder, CO 80301 (303) 473-0388 x132 http://www.miltope.com <http://www.miltope.com/>=20 =20 =20 ************************************************************************ **** ** CONFIDENTIALITY NOTE: This email and any attachments may be confidential=20 and protected by legal privilege. If you are not the intended recipient,=20 be aware that you are hereby notified that any unauthorized review, use, disclosure, dissemination, distribution, or copying of this communication,=20 or any of its contents, is strictly prohibited. If you have received this=20 email in error, please notify us immediately by replying to the sender and=20 deleting this copy and the reply from your system. Thank you for your cooperation. ************************************************************************ **** ** ------------------------------------------------------------------ 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: =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 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: =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 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