# [SI-LIST] Re: Diff.Pairs

• From: Doug Brooks <doug@xxxxxxxxxx>
• To: si-list@xxxxxxxxxxxxx
• Date: Fri, 03 Oct 2003 12:36:36 -0700

```Tight may be a relative word. But a differential pair constitutes a "loop"
in EMI terms. That is, the loop is the area encompassed by the signal and
its return. Smaller loop areas perform better than larger loop areas when
EMI is a concern. The closer the differential pair, the smaller is the
loop. If we are NOT concerned about EMI, then this is not an issue. If we
ARE, then we might want to pay attention to this and keep the loop small by
routing the traces close together.

The equal spacing "requirement" comes from the control of reflections (ie
transmission line termination issues.) IF we are concerned about
reflections, THEN we need a constant impedance everywhere along the trace.
IF the (differential) traces are close together (for EMI reasons) THEN they
will interact (a very special case of crosstalk, which in this particular
case [signals --- being equal and opposite --- are exactly correlated with
each other] is not a problem.) IF we want to keep a constant impedance
along the traces, THEN we must keep a "constant" spacing between them,
because the coupling between them, and therefore the differential
impedance, will vary if we don't.

There is a further design rule you sometimes hear, that being that the
differential traces must be equal length. This is NOT for timing reasons,
but for common mode reasons. A strong assumption we make about differential
signals is that they are equal and opposite, and therefore there is no
return signal through the ground system. Even if the signals are perfect,
if the traces are different length, then the signal will not arrive at the
far end at exactly the same time and the signals will not be "equal and
opposite" at the receiver. Just a couple of degrees phase shift can make a
surprising difference between the signals when we are talking about
(square-wave) clock signals. If the signals are not exactly equal and
opposite, then there MUST be a net current flowing somewhere else. This
will quite likely be a common mode noise current that might cause an EMI issue.

None of the differential signal trace design rules are necessary taken by
themselves. This is important to recognize. But if are concerned about
certain SI issues, they might lead to some design considerations which THEN
might cascade (like a domino effect) into other areas.

This is in my book, too...............

Doug Brooks

At 11:41 AM 10/3/2003 -0700, Lee Ritchey wrote:
>More than that, it does not have any benefit.  Tight coupling of
>differential pairs forces the traces to be narrower increasing the skin
>effect losses.  Also, this tight coupling is going to result in good old
>cross talk that actually degrades the edges.
>
>How the notion of tight coupling of differential pairs as beneficial got
>started is a mystery to me.  There are several references that show that
>tight coupling is not beneficial, one of them is Howard Johnson's latest
>book, at least one column he has written and my recently released book.
>
>Lee Ritchey
>
>
> > [Original Message]
> > From: Duane Takahashi <duanet@xxxxxxxxxxxxxxxxxxxxxx>
> > To: <si-list@xxxxxxxxxxxxx>
> > Date: 10/2/2003 3:58:59 PM
> > Subject: [SI-LIST] Re: Diff.Pairs
> >
> > Hi Juergen:
> >
> > Aligning the stack up for the broadside coupled diff lines is expensive.
> >    You can do this, but it drives up the cost of the board.
> >
> > Duane
> >
> > > Hi Juergen,
> > > You can find lots of  application notes
> > > especially with respect to process variation
> > > on differential pairs here:
> > >
> > >
> > > www.polarinstruments.com/support/cits/cits_index.html
> > >
> > > In particular this one may be of interest:
> > >
> > >
> > > How measured impedance may vary from field solver calculations when
> > > using woven glass reinforced
> > > <http://www.polarinstruments.com/support/cits/AP139.html>laminates
> > >
> > > www.polarinstruments.com/support/cits/AP139.html
> > >
> > >
> > > And this note:
> > >
> > > Copper thickness, edge coupled lines and
> > > characteristic
> > > <http://www.polarinstruments.com/support/cits/AP151.html>impedance
> > >
> > >
> > > www.polarinstruments.com/support/cits/AP151.html
> > >
> > >
> > >
> > > Hope this helps....
> > >
> > >
> > > Kind regards
> > > Martyn Gaudion
> > > www.polarinstruments.com
> > > T: +44 1481 253081
> > > F: +44 1481 252476
> > > M: +44 7710 522748
> > > E: martyn@xxxxxxxxxxxxxxxxxxxx
> > >
> > > ============================================
> > >   Controlled Impedance & Signal integrity tools
> > >   for the Printed circuit fabrication industry
> > > ============================================
> > >
> > >
> > >
> > >
> > >
> > >
> > > At 19:00 02/10/2003, you wrote:
> > >
> > >>I am seeking help in finding enlightenment regarding electrical
> > >>performance pros and cons and how manufacturing tolerances play a role
> > >>when comparing side by side and tandem differential pairs. I'd
>appreciate
> > >>your opinion, experience, analysis, pointers to papers and articels,
>etc.
> > >>
> > >>In return, I would offer to share a summary of the finding/discoveries
> > >>with interested parties.
> > >>
> > >>Thanks
> > >>
> > >>Juergen
> > >>
> > >>
> > >>
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> > >
> > >
> > >
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> >
> > --
> > Duane Takahashi              phone: 408-720-4200
> > Greenfield Networks            fax: 408-720-4210
> > 255 Santa Ana Court          email: duanet@xxxxxxxxxxxxxxxxxxxxxx
> > Sunnyvale, CA 94085
> >
> >
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>
>
>
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Doug Brooks' new book, "Signal Integrity Issues and Printed Circuit Board
Design" has just been released by Prentice Hall. See details and ordering
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