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

• From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
• To: <si-list@xxxxxxxxxxxxx>
• Date: Wed, 30 Nov 2005 14:21:20 -0800

```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=7 mils, Width=7 mils, Separation (edge to edge) = 15
mils, Z0=64 ohms, on an FR-4 type substrate.
The crossover point (where NEXT=FEXT) 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 =
stripline I would expect (due to cancellation) NEXT to be
greater than =
FEXT.

BUT!

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

Aubrey Sparkman=20
Enterprise Engineering Signal Integrity Team
Dell, Inc.=20
Aubrey_Sparkman@xxxxxxxx=20
(512) 723-3592

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] =
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=A1=AAA
Handbook of =
Interconnect Theory and Design Practices", you can find that
the =
inductive and capacitive crosstalk cancel at far end and
reinforce at =
near end.

You can also get this from section 5.7.1-5.7.3 in
"High-speed digital =
design, Dr. Johnson".
=20
So it's possible to have NEXT greater than FEXT.

B.R.
Astrom

--- "Grasso, Charles"
<Charles.Grasso@xxxxxxxxxxxx>=D0=B4=B5=C0:

> Is it possible to have NEXT greater than FEXT.??
> =20
>=20
> For example: For a two trace microstrip system my
simulator reports=20
> 10% NEXT (so for a 2.5V signal that translates to 250mV).
>=20
> Based on the reported mutuals and self numbers and using
the formula=20
> in Eric's book I estimate 60mV for FEXT.
>=20
> =20
>=20
> Is this reasonable?
>=20
> =20
>=20
> 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
>=20
> =20
>=20
>=20
>
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