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

Well put!
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: Scott McMorrow 
To: leeritchey@xxxxxxxxxxxxx
Cc: Dr. Howard Johnson; si-list@xxxxxxxxxxxxx
Sent: 11/30/2005 4:02:49 PM 
Subject: Re: [SI-LIST] Re: NEXT and FEXT: Question on relative levels


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

      
------------------------------------------------------------
------
    
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the
      
Subject field
    
=20
or to administer your membership from a web page, go
to:
http://www.freelists.org/webpage/si-list
=20
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject
      
field
    
=20
List FAQ wiki page is located at:
              =20
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
=20
List technical documents are available at:
                http://www.si-list.org
=20
List archives are viewable at:    =20
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
 =20
=20
=20
      

=09

=09
        =09
___________________________________________________________
=D1=C5=BB=A2=C3=E2=B7=D1G=D3=CA=CF=E4=A3=AD=D6=D0=B9=FA=B5=D
A=D2=BB=BE=F8=
=CE=DE=C0=AC=BB=F8=D3=CA=BC=FE=C9=A7=C8=C5=B3=AC=B4=F3=D3=CA
=CF=E4
http://cn.mail.yahoo.com

------------------------------------------------------------
------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the
Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject
field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:    =20
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
 =20
------------------------------------------------------------
------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the
Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject
field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu


------------------------------------------------------------
------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the
Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject
field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  
    



------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

  

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: