[SI-LIST] Re: NEXT and FEXT: Question on relative levels

OK, Charles, I've had less coffee today and perhaps thought
of a better way to put it.  How about this: for a microstrip
configuration there is no fixed rule for whether FEXT or
NEXT is bigger. Either can exceed the other under
appropriate circumstances.

I liked the submissions that followed mine pointing out that
of course the NEXT (crosstalk initially directed toward the
source) often bounces at the source, ending up at the far
end of the line anyway.

To those comments I should like to add that if two parallel
lines are oriented in opposite directions, then the NEXT
from the aggressor points straight back into the receiver of
the victim.

When I want to understand the problem I go to the trouble of
working through all these fiddly cases, bu when I want a
numerical answer I think a crosstalk simulator is a good
idea.

Did that help or did I make things worse?

Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505,  howie03@xxxxxxxxxx
http:\\sigcon.com  -- High-Speed Digital Design seminars,
publications and films


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Grasso,
Charles
Sent: Friday, December 02, 2005 8:46 AM
To: howie03@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: NEXT and FEXT: Question on relative
levels


Hi Howard,

First, thank you for your extensive replies. However I will
admit to
being slightly confused on your initial statements.

In Email one you wrote:" I will make the claim that NEXT can
and does
often exceed FEXT."

Then in a follow up email you corrected that (I think) by
saying: "Well,
that's just like me to get something backwards...

May I restate what I MEANT to say?  "I will make the claim
that FEXT can
and does often exceed NEXT".... all else seems fine in this
message.
Sorry for the goof"=20

OK so far...If I understand you correctly then one can have
FEXT
exceeding NEXT especially in non-homogenous applications.

Now Email 3: ..  anyway we are discussing cancellation of
far-end-crosstalk FEXT and whether therefore in a microstrip
the
near-end-crosstalk NEXT can exceed (did I get it right this
time?) the
FEXT, and I said yes, it can and does in many cases.

Email 3 seems to suggest that NEXT exceeds FEXT (consistent
with email
1).

Can you please help me understand this better.??

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 Dr. Howard Johnson
Sent: Wednesday, November 30, 2005 4:30 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: NEXT and FEXT: Question on relative
levels

Hi David,

I will comment on your question, if you can excuse my
previous confusion -- having one of those days, I guess,
something about the construction of this question just seems
to be getting to me..., and I replied to quickly to someone
who had written to me...  anyway we are discussing
cancellation of far-end-crosstalk FEXT and whether therefore
in a microstrip the near-end-crosstalk NEXT can exceed (did
I get it right this time?) the FEXT, and I said yes, it can
and does in many cases.

I do not have any information about the relative rms values,
but as far as I can tell the AREAS under the FEXT and NEXT
curves seem to hold a constant ratio that is determined by
the geometry of the transmission structure. This ratio does
NOT vary with line length or risetime. The AREA for both
effects grows in direct proportion to the length of the
line.

I should mention also that the FEXT amplitude does not grow
forever with increasing length. If you project a
transmission line all the way to Pluto, with FEXT growing
strictly in proportion to length, the predicted FEXT
amplitude would grow unreasonably large; what seems to
happen instead is that the FEXT progresses to a level of 50%
after which you see the duration of the FEXT effect begin to
extend, much as with NEXT.

Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505,  howie03@xxxxxxxxxx
http:\\sigcon.com  -- High-Speed Digital Design seminars,
publications and films


-----Original Message-----
From: David Siadat (dsiadat) [mailto:dsiadat@xxxxxxxxx]
Sent: Wednesday, November 30, 2005 2:35 PM
To: howie03@xxxxxxxxxx
Subject: RE: [SI-LIST] Re: NEXT and FEXT: Question on
relative levels


Dr. Johnson,

Would you comment on the difference between peak versus rms
noise of the
example you have shown.
I was wondering if the area under each (NEXT and FEXT) plot
has any
merit in noise comparison?

Regards,
David

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Dr. Howard Johnson
Sent: Wednesday, November 30, 2005 2:21 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: NEXT and FEXT: Question on relative
levels

To Charles et. al.,

I will make the claim that NEXT can and does often exceed
FEXT.  In a
homogeneous dielectric (same Er all around), with a uniform
cross-section trace I believe you always get perfect
cancellation of
FEXT.  When you mix some air into the configuration (as in a
microstrip)
it weakens the capacitive coupling and you always get (as
far as I know)
a negative FEXT coefficient. This last statement is an
assertion based
on experience and measurement, but not something for which I
have a
proof. I would be grateful to anyone who can point me to a
solid proof
of that assertion.

In ordinary pcb applications, if you plot crosstalk
amplitude versus
line length, the NEXT and FEXT waveforms start out with NEXT
biggest,
and they keep the same ratio as they both grow in proportion
to line
length. Then, once the line becomes long enough so that the
roundtrip
delay exceeds the signal rise/fall time (this is a "critical
length" for
the NEXT/FEXT problem), NEXT stops growing in amplitude but
instead
extends in duration.  FEXT keeps growing in amplitude,
eventually
exceeding the NEXT amplitude.

I have an example pictures here:
www.sigcon.com/Pubs/misc/TemporaryPicture.gif
The picture was computed with Hyperlynx linesim. I show this
picture,
and some others like it, in my High-Speed Digital Design
seminar (next
one is in January 2006).

The picture shows both NEXT and FEXT waveforms (measured at
opposite
ends of a victim line with terminations at both ends), for
various
lengths of 1/2, 1, 2, 4, 8 and 16 inches.
The signal risetime is 1 ns.
The microstrip trace geometry appears in the picture.
Height=3D7 mils, Width=3D7 mils, Separation (edge to edge)
=3D 15
mils, Z0=3D64
ohms, on an FR-4 type substrate.
The crossover point (where NEXT=3DFEXT) for this particular
setup occurs
at about 11 inches length. Crosstalk of either type has a
peak amplitude
of about 8% at that point.  Below this critical length, NEXT
is bigger.

A faster risetime would exaggerates the FEXT, making it
larger compared
to NEXT and moving the crossover point to a shorter
distance.  For
example, with a 100-ps rise/fall time the crossover would
happen at
about 1 inch, with crosstalk of both types at 8% at that
risetime and
length.

Comments?

Best regards,
Dr. Howard Johnson, Signal Consulting Inc., tel +1
509-997-0505,
howie03@xxxxxxxxxx http:\\sigcon.com  -- High-Speed Digital
Design
seminars, publications and films


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
Aubrey_Sparkman@xxxxxxxx
Sent: Wednesday, November 30, 2005 8:05 AM
To: chengyuming_ah@xxxxxxxxxxxx;
Charles.Grasso@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: ??: NEXT and FEXT: Question on
relative levels


Let's be a little more clear about the situation we are
discussing.  For
=3D stripline I would expect (due to cancellation) NEXT to
be
greater than
=3D FEXT.

BUT!

Charles asked about two MICROSTRIP traces.  Anyone want to
claim the =3D
inductive and capacitive coupling for microstrip traces is
sufficiently
=3D balanced for cancellation to apply?

Aubrey Sparkman=3D20
Enterprise Engineering Signal Integrity Team Dell, Inc.=3D20
Aubrey_Sparkman@xxxxxxxx=3D20
(512) 723-3592

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] =3D
On Behalf Of Yuming Cheng
Sent: Tuesday, November 29, 2005 11:29 PM
To: Charles.Grasso@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] ??: NEXT and FEXT: Question on relative
levels

Hello Charles,

>From figure 3.2 of "High-Speed Digital System
Design=3DA1=3DAAA
Handbook of =3D
Interconnect Theory and Design Practices", you can find that
the =3D
inductive and capacitive crosstalk cancel at far end and
reinforce at =3D
near end.

You can also get this from section 5.7.1-5.7.3 in
"High-speed digital =3D
design, Dr. Johnson".
 =3D20
So it's possible to have NEXT greater than FEXT.

B.R.
Astrom

--- "Grasso, Charles"
<Charles.Grasso@xxxxxxxxxxxx>=3DD0=3DB4=3DB5=3DC0:

> Is it possible to have NEXT greater than FEXT.??
> =3D20
>=3D20
> For example: For a two trace microstrip system my
simulator reports=3D20
> 10% NEXT (so for a 2.5V signal that translates to 250mV).
>=3D20
> Based on the reported mutuals and self numbers and using
the formula=3D20
> in Eric's book I estimate 60mV for FEXT.
>=3D20
> =3D20
>=3D20
> Is this reasonable?
>=3D20
> =3D20
>=3D20
> 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
>=3D20
> =3D20
>=3D20
>=3D20
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