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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: leeritchey@xxxxxxxxxxxxx
  • Date: Wed, 30 Nov 2005 19:02:42 -0500

Lee
I'd like to follow on to your post and actually define the terminology.

Forward Crosstalk - the forward traveling crosstalk wave produced by 
coupled transmission lines.
Reverse Crosstalk - the reverse traveling crosstalk wave produced by 
coupled transmission lines.

    Forward and reverse are defined with respect to the direction of the
    aggressor wavefront.

Near End Crosstalk (NEXT) - is the measured crosstalk at the near end of 
a victim line.
Far End Crosstalk (FEXT) - is the measured crosstalk at the far end of a 
victim line.

    Near end and far end are defined with respect to the location of the
    excitation port.

What Dr. Johnson and most others have been discussing is Forward and 
Reverse crosstalk.  However, as Lee points out, what we actually see in 
a system is Near End and Far End crosstalk ... AND ... this is colored 
by other transmission line phenonema, such as reflections off of the 
drivers and receivers.  What you "see" depends upon the impedance of the 
drivers and receivers at each end of each line.  And, quite frankly, 
except for the absolutely perfectly terminated case, the wave phenonema 
become quite complex.  For example, in a 2-line coupled system, the 
following can happen:

    * Reverse crosstalk can be reflected off of a low impedance driver
      with a negative reflection coefficient and travel in the forward
      direction, becoming part of the FEXT contribution, and can, in
      fact, be mistaken for Forward Crosstalk.
    * High impedance receivers reflect received crosstalk with a
      positive reflection coefficient of 1, thereby doubling the
      magnitude of FEXT.
    * High impedance receiver reflections of aggressor waveforms change
      the actual direction of the forward waveform, thereby creating a
      new set of forward and reverse crosstalk components, which then
      reflect and become part of the FEXT and NEXT contribution.
    * And on, and on.

A very long time ago, I tried to use my head to understand a simple 
2-line coupled system, with various driver and receiver terminations.  
Finally, after convincing myself that my simulator was doing the right 
thing, I gave up.

regards

Scott



Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



Lee Ritchey wrote:

>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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