Charles, Sizeable means perhaps 2:1 - Eugene, please help me out here, since you did the measurement. And, I am sorry, I am not the one to validate a measurement with a field solver. I usually validate field solvers with measurement :-). 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 Grasso, Charles Sent: Monday, April 24, 2006 11:32 AM To: Smolyansky, Dmitry A; aleks@xxxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Cc: Mayevskiy, Eugene Subject: [SI-LIST] Differential pair Impedance - Q on test data Hi Dima, I was revisiting the diff pair email thread and ran across an interesting statement that you made regarding diff pairs and their pwb implementation. In your reply you said:=3D20 " 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." You are right. I am VERY surprised. I have a couple of questions for you.. a) What do you mean by sizable ratio? b) Did you correlate the data with a 2D solver? Best Regards Charles Grasso Senior Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Pager/Short Message: 3032042974@xxxxxxxx Email: charles.grasso@xxxxxxxxxxxx; Email Alternate: chasgrasso@xxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of dmitry.a.smolyansky@xxxxxxxxxxxxxx Sent: Thursday, April 20, 2006 11:22 AM To: aleks@xxxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Cc: eugene.mayevskiy@xxxxxxxxxxxxxx Subject: [SI-LIST] Re: Question regarding return current in a differential pair 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, =3D3D20 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? =3D3D20 Thanks in advance, Mike =3D3D20 Michael Kotson Project Engineer VT Miltope=3D3D20 4900 Pearl East Circle Suite 106 Boulder, CO 80301 (303) 473-0388 x132 http://www.miltope.com <http://www.miltope.com/>=3D3D20 =3D3D20 =3D3D20 ************************************************************************ **** ** CONFIDENTIALITY NOTE: This email and any attachments may be confidential=3D3D20 and protected by legal privilege. If you are not the intended recipient,=3D3D20 be aware that you are hereby notified that any unauthorized review, use, disclosure, dissemination, distribution, or copying of this communication,=3D3D20 or any of its contents, is strictly prohibited. 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