[SI-LIST] Re: Differential pair Impedance - Q on test data

If the spacing to ground is kept constant while changing the spacing between
the lines the result is not surprising at all because the differential
impedance of two tracks is primarily dictated by the distance to the
reference planes and the line spacing has only a secondary effect. If
otherwise I'd recommend revisiting those measurements
Regards

Ihsan


On 4/24/06, Grasso, Charles <Charles.Grasso@xxxxxxxxxxxx> wrote:
>
> 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:=20
> " 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,
> =3D20
> 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?
>
> =3D20
>
> Thanks in advance,
>
> Mike
>
> =3D20
>
> Michael Kotson
>
> Project Engineer
>
> VT Miltope=3D20
>
> 4900 Pearl East Circle Suite 106
>
> Boulder, CO 80301
>
> (303) 473-0388 x132
>
> http://www.miltope.com <http://www.miltope.com/>=3D20
>
> =3D20
>
> =3D20
>
>
>
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