[SI-LIST] Re: Differential pair Impedance - Q on test data
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: "Ihsan Erdin" <erdinih@xxxxxxxxx>
- Date: Mon, 24 Apr 2006 15:09:13 -0600
Ishan,
The "secondary effect" you are referring to can easily throw off a
design.
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
________________________________
From: Ihsan Erdin [mailto:erdinih@xxxxxxxxx]
Sent: Monday, April 24, 2006 2:10 PM
To: Grasso, Charles
Cc: dmitry.a.smolyansky@xxxxxxxxxxxxxx; aleks@xxxxxxxxxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx; eugene.mayevskiy@xxxxxxxxxxxxxx
Subject: Re: [SI-LIST] 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
<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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