[SI-LIST] Re: Question regarding return current in a differential pair

  • From: <dmitry.a.smolyansky@xxxxxxxxxxxxxx>
  • To: <aleks@xxxxxxxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 20 Apr 2006 10:21:32 -0700

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



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