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

  • From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
  • To: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Wed, 30 Nov 2005 15:41:53 -0800

That's pretty much how it works except in logic circuits, the driver of the
victim line is usually set at a 1 or a 0 which looks line a short circuit. 
In this case, the NEXT reflects off this short and appears at the far end,
looking for all the world like FEXT.  Moreover, if that end of the line is
an open circuit, it doubles!  Not what most expect to see.

Lee W. Ritchey
Speeding Edge
P. O. Box 2194
Glen Ellen, CA 95442
Phone- 707-568-3983
FAX-    707-568-3504

I just used the energy it took to be angry to write some blues.
Count Basie


> [Original Message]
> From: Dr. Howard Johnson <howie03@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 11/30/2005 3:25:51 PM
> Subject: [SI-LIST] Re: NEXT and FEXT: Question on relative levels
>
> 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.
>
> -----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=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 exaggerate 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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