[SI-LIST] Re: Diff.Pairs

  • From: "Chris McGrath" <chris.mcgrath@xxxxxxxx>
  • To: "silist" <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 9 Oct 2003 12:04:08 -0700

My experience has generally been that routing the differential pairs
close together has the benefit of allowing the designer to increase
routing density and can be routed closer together than two single ended
signals.  I am very interested in understanding if there are benefits of
a tightly coupled differential pair where the electromagnetic coupling
yields better SI or EMI (or other) characteristics.

-Chris


> -----Original Message-----
> From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]=20
> Sent: Thursday, October 09, 2003 2:41 PM
> To: leeritchey@xxxxxxxxxxxxx; silist
> Subject: [SI-LIST] Re: Diff.Pairs
>=20
>=20
> Lee,
> You said about differntial pair nets:
>=20
>     "unless they are placed close enough together that there is some
>     interaction. This interaction is not beneficial."
>=20
> You have yet to prove that the coupled interaction of closely spaced=20
> differential pairs is not beneficial.  Please show the proof and the=20
> data.  Analytically, electromagnetically and observationally there is=20
> not a problem with using close spaced differential pairs.   You are=20
> stating your opinion as an absolute fact that is not supported by=20
> science.  This is quite contrary to your stated belief in good=20
> engineering and science.
>=20
> scott
>=20
>=20
> Lee Ritchey wrote:
>=20
> >Why do people try so hard to make the coupling between the=20
> two members=20
> >of a pair so important.  These are two independent signals=20
> that travel=20
> >over planes independently unless they are placed close=20
> enough together=20
> >that there is some interaction.  This interaction is not beneficial.
> >
> >Lee
> >
> >
> > =20
> >
> >>[Original Message]
> >>From: Knighten, Jim L <JK100005@xxxxxxxxxxxxxxxx>
> >>To: <leeritchey@xxxxxxxxxxxxx>; Doug Brooks <doug@xxxxxxxxxx>;
> >>   =20
> >>
> ><si-list@xxxxxxxxxxxxx>
> > =20
> >
> >>Date: 10/9/2003 10:40:29 AM
> >>Subject: RE: [SI-LIST] Re: Diff.Pairs
> >>
> >>Lee,
> >>
> >>Your post is interesting!
> >>
> >>Differential signaling is usually implemented with coupled=20
> >>transmission lines.  The mutual coupling between the traces=20
> affect the=20
> >>two modes that
> >>   =20
> >>
> >are
> > =20
> >
> >>always present (even and odd modes). In the traditional=20
> configuration,=20
> >>the two traces are parallel and of the same width and thickness and=20
> >>located adjacent to a plane.  The degree of coupling between the=20
> >>traces is usually described as "loosely coupled" or=20
> "tightly coupled." =20
> >>In either case, if
> >>   =20
> >>
> >the
> > =20
> >
> >>signal and signal traces are perfectly differential (i.e., no=20
> >>imbalance, perfectly symmetrical), then there is always=20
> current in the=20
> >>adjacent
> >>   =20
> >>
> >ground
> > =20
> >
> >>plane, but the net current in the longitudinal direction (the=20
> >>direction of the traces) is zero.  The currents that exist in the=20
> >>adjacent plane are circulating currents that reflect the=20
> distributed=20
> >>coupling between the traces down the length of the=20
> transmission line.
> >>
> >>So, what if the two coupled traces are not co-planar, i.e.,=20
> not in the
> >>   =20
> >>
> >same
> > =20
> >
> >>plane?  Well, you still have two coupled transmission lines, but the
> >>   =20
> >>
> >mutual
> > =20
> >
> >>capacitance and inductance between them may be different=20
> than if they=20
> >>were co-planar, hence the even and odd mode impedances may be=20
> >>different.  These non-co-planar coupled lines can still carry=20
> >>differential signals, though.
> >>   =20
> >>
> >
> > =20
> >
> >>What if the two coupled lines were not co-planar and=20
> actually had the
> >>   =20
> >>
> >ground
> > =20
> >
> >>plane between them?  This is just a special case of the "loosely=20
> >>coupled" case, in that the lines are now not coupled at=20
> all.  Still,=20
> >>the lines can support differential signaling, but the relationships=20
> >>between even and odd modes are not quite the same as when they were=20
> >>coupled. (Perhaps even mode and odd mode impedances are equal?)
> >>
> >>So, how about current in the ground plane?  For perfect=20
> differential=20
> >>signaling, the net current in the plane is zero.  When you=20
> introduce=20
> >>imbalance, either in the signal source, or in the signal path, you=20
> >>create net longitudinal current in the ground plane.  This=20
> is the even=20
> >>mode
> >>   =20
> >>
> >signal,
> > =20
> >
> >>which has no bearing on your intended differential signal (the odd=20
> >>mode)
> >>   =20
> >>
> >and
> > =20
> >
> >>represents an EMI source on the ground plane.
> >>
> >>If you route differential signals on different layers, it=20
> may be more=20
> >>difficult to maintain balance (symmetry) in the traces than if the=20
> >>traces were co-planar.  If this is true, you have more=20
> potential for=20
> >>EMI issues.
> >>
> >>...My thoughts
> >>
> >>Jim
> >>
> >>________________________
> >>James L. Knighten, Ph.D.
> >>Teradata, a division of NCR                 http://www.ncr.com
> >>17095 Via del Campo
> >>San Diego, CA 92127
> >>tel: 858-485-2537
> >>fax: 858-485-3788
> >>
> >>
> >>-----Original Message-----
> >>From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> >>Sent: Thursday, October 09, 2003 9:30 AM
> >>To: Doug Brooks; si-list@xxxxxxxxxxxxx
> >>Subject: [SI-LIST] Re: Diff.Pairs
> >>
> >>If this discussion is about differential pairs travelling over the=20
> >>planes of a PCB, the return current for each member of the pair=20
> >>travels on the plane over which it travels, not on the=20
> other wire.  If=20
> >>they are very tightly coupled to each other, perhaps 5% of=20
> the current=20
> >>from one travels in the other.  It is coincidental that the two=20
> >>currents are equal in magnitude and opposite.  They don't=20
> have to be. =20
> >>Their "return currents" still travel on the plane, not on the other=20
> >>wire.
> >>
> >>As far as EMI is concerned, it has been demonstrated many=20
> times, once=20
> >>in the paper done by Doug Brooks with the staff at UMR, that traces=20
> >>traveling over planes are not a detectable source of EMI. =20
> Therefore,=20
> >>constraining the routing of differential pairs to prevent them from=20
> >>creating EMI is not appropriate or necessary.
> >>
> >>It is still true that the two members of a differential=20
> pair are two=20
> >>independent signals traveling on two independent=20
> transmission lines.=20
> >>All they have in common is that the have equal amplitudes=20
> and are 180=20
> >>degrees out of phase with each other.  If the protocol is=20
> LVDS, each=20
> >>member of the pair should be parallel terminated in an=20
> impedance equal=20
> >>to Zo for that line to Vref (about 1.25V) which is half way between=20
> >>the two logic levels.
> >>
> >>As long as the two signals switch at the same time, the current=20
> >>flowing
> >>   =20
> >>
> >out
> > =20
> >
> >>of Vref into one line is the same magnitude an opposite in=20
> polarity to
> >>   =20
> >>
> >that
> > =20
> >
> >>flowing into the other.  The net current into and out of the Vref=20
> >>terminal is zero, so we can omit the connection.  When we=20
> do this, we=20
> >>have two resistors, each of value Zo across the ends of the two=20
> >>transmission
> >>   =20
> >>
> >lines.
> > =20
> >
> >>For convenience, we use one resistor of value 2 X Zo.  This=20
> is not a=20
> >>differential impedance of 100 ohms, but two parallel=20
> terminations of=20
> >>value Zo terminating two transmission lines each of impedance Zo.
> >>
> >>As long as the two edges switch at the same time, there is=20
> no current=20
> >>imbalance and all is well.  Soon as one edge switches before the=20
> >>other, there is a need for a momentary current spike to=20
> flow into or=20
> >>out of the Vref terminal.  If there is no connection to=20
> Vref for the=20
> >>current flow,
> >>   =20
> >>
> >the
> > =20
> >
> >>result is the edges are degraded.  To avoid this=20
> degradation, a very=20
> >>small capacitor is often connected between the two resistors and=20
> >>ground.  This
> >>   =20
> >>
> >is
> > =20
> >
> >>a very common termination for 2.4 GB/S signal links.
> >>
> >>It is time to stop representing differential signals as=20
> needing to be=20
> >>tightly coupled to each other in order to operate properly.  It is=20
> >>simply not so.  I have routed thousands of differential=20
> signal where=20
> >>each member of the pair is on a different layer.  If this were not=20
> >>possible, 1 mm
> >>   =20
> >>
> >pitch
> > =20
> >
> >>BGAs with differential signals would be un routable.  There=20
> are tens=20
> >>of thousands of such parts being shipped every month on PCBs where=20
> >>they are routed apart from each other.
> >>
> >>This is all described in my recently published book, "Right=20
> the First
> >>   =20
> >>
> >Time,
> > =20
> >
> >>A Practical Handbook on High Speed PCB and System Design". =20
> It is also=20
> >>covered in Howard Johnson's new book whose title escapes me at the
> >>   =20
> >>
> >moment..
> > =20
> >
> >>Lee
> >>
> >>
> >>   =20
> >>
> >>>[Original Message]
> >>>From: Doug Brooks <doug@xxxxxxxxxx>
> >>>To: <si-list@xxxxxxxxxxxxx>
> >>>Date: 10/3/2003 1:02:25 PM
> >>>Subject: [SI-LIST] Re: Diff.Pairs
> >>>
> >>>Tight may be a relative word. But a differential pair constitutes a
> >>>     =20
> >>>
> >>"loop"
> >>   =20
> >>
> >>>in EMI terms. That is, the loop is the area encompassed by=20
> the signal
> >>>     =20
> >>>
> >and
> > =20
> >
> >>>its return. Smaller loop areas perform better than larger=20
> loop areas
> >>>     =20
> >>>
> >when
> > =20
> >
> >>>EMI is a concern. The closer the differential pair, the smaller is=20
> >>>the
> >>>loop. If we are NOT concerned about EMI, then this is not=20
> an issue. If
> >>>     =20
> >>>
> >we
> > =20
> >
> >>>ARE, then we might want to pay attention to this and keep the loop=20
> >>>small
> >>>     =20
> >>>
> >>by
> >>   =20
> >>
> >>>routing the traces close together.
> >>>
> >>>The equal spacing "requirement" comes from the control of=20
> reflections
> >>>     =20
> >>>
> >(ie
> > =20
> >
> >>>transmission line termination issues.) IF we are concerned about
> >>>reflections, THEN we need a constant impedance everywhere along the
> >>>     =20
> >>>
> >>trace.
> >>   =20
> >>
> >>>IF the (differential) traces are close together (for EMI reasons)=20
> >>>THEN
> >>>     =20
> >>>
> >>they
> >>   =20
> >>
> >>>will interact (a very special case of crosstalk, which in this
> >>>     =20
> >>>
> >particular
> > =20
> >
> >>>case [signals --- being equal and opposite --- are exactly=20
> correlated
> >>>     =20
> >>>
> >>with
> >>   =20
> >>
> >>>each other] is not a problem.) IF we want to keep a constant=20
> >>>impedance
> >>>along the traces, THEN we must keep a "constant" spacing=20
> between them,=20
> >>>because the coupling between them, and therefore the differential=20
> >>>impedance, will vary if we don't.
> >>>
> >>>There is a further design rule you sometimes hear, that being that=20
> >>>the
> >>>differential traces must be equal length. This is NOT for timing
> >>>     =20
> >>>
> >reasons,
> > =20
> >
> >>>but for common mode reasons. A strong assumption we make about
> >>>     =20
> >>>
> >>differential
> >>   =20
> >>
> >>>signals is that they are equal and opposite, and therefore=20
> there is=20
> >>>no
> >>>return signal through the ground system. Even if the signals are
> >>>     =20
> >>>
> >perfect,
> > =20
> >
> >>>if the traces are different length, then the signal will=20
> not arrive=20
> >>>at
> >>>     =20
> >>>
> >>the
> >>   =20
> >>
> >>>far end at exactly the same time and the signals will not=20
> be "equal=20
> >>>and
> >>>opposite" at the receiver. Just a couple of degrees phase=20
> shift can make
> >>>     =20
> >>>
> >>a
> >>   =20
> >>
> >>>surprising difference between the signals when we are talking about
> >>>(square-wave) clock signals. If the signals are not=20
> exactly equal and=20
> >>>opposite, then there MUST be a net current flowing=20
> somewhere else. This=20
> >>>will quite likely be a common mode noise current that=20
> might cause an EMI
> >>>     =20
> >>>
> >>issue.
> >>   =20
> >>
> >>>None of the differential signal trace design rules are necessary=20
> >>>taken
> >>>     =20
> >>>
> >by
> > =20
> >
> >>>themselves. This is important to recognize. But if are concerned=20
> >>>about
> >>>certain SI issues, they might lead to some design=20
> considerations which
> >>>     =20
> >>>
> >>THEN
> >>   =20
> >>
> >>>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:
> >>>     =20
> >>>
> >>>>More than that, it does not have any benefit.  Tight coupling of=20
> >>>>differential pairs forces the traces to be narrower=20
> increasing the=20
> >>>>skin effect losses.  Also, this tight coupling is going=20
> to result in=20
> >>>>good
> >>>>       =20
> >>>>
> >old
> > =20
> >
> >>>>cross talk that actually degrades the edges.
> >>>>
> >>>>How the notion of tight coupling of differential pairs as=20
> beneficial
> >>>>       =20
> >>>>
> >got
> > =20
> >
> >>>>started is a mystery to me.  There are several references=20
> that show
> >>>>       =20
> >>>>
> >that
> > =20
> >
> >>>>tight coupling is not beneficial, one of them is Howard Johnson's
> >>>>       =20
> >>>>
> >latest
> > =20
> >
> >>>>book, at least one column he has written and my recently released=20
> >>>>book.
> >>>>
> >>>>Lee Ritchey
> >>>>
> >>>>
> >>>>       =20
> >>>>
> >>>>>[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
> >>>>>         =20
> >>>>>
> >>expensive.
> >>   =20
> >>
> >>>>>   You can do this, but it drives up the cost of the board.
> >>>>>
> >>>>>Duane
> >>>>>
> >>>>>         =20
> >>>>>
> >>>>>>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
> >>>>>>           =20
> >>>>>>
> >when
> > =20
> >
> >>>>>>using woven glass reinforced=20
> >>>>>><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=20
> >>>>>><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
> >>>>>>
> =
>>>>>>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> >>>>>>  Controlled Impedance & Signal integrity tools
> >>>>>>  for the Printed circuit fabrication industry=20
> =
>>>>>>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>At 19:00 02/10/2003, you wrote:
> >>>>>>
> >>>>>>           =20
> >>>>>>
> >>>>>>>I am seeking help in finding enlightenment regarding=20
> electrical=20
> >>>>>>>performance pros and cons and how manufacturing=20
> tolerances play a
> >>>>>>>             =20
> >>>>>>>
> >>role
> >>   =20
> >>
> >>>>>>>when comparing side by side and tandem differential pairs. I'd
> >>>>>>>             =20
> >>>>>>>
> >>>>appreciate
> >>>>       =20
> >>>>
> >>>>>>>your opinion, experience, analysis, pointers to papers and
> >>>>>>>             =20
> >>>>>>>
> >articels,
> > =20
> >
> >>>>etc.
> >>>>       =20
> >>>>
> >>>>>>>In return, I would offer to share a summary of the
> >>>>>>>             =20
> >>>>>>>
> >>finding/discoveries
> >>   =20
> >>
> >>>>>>>with interested parties.
> >>>>>>>
> >>>>>>>Thanks
> >>>>>>>
> >>>>>>>Juergen
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>-------------------------------------------------------
> ----------
> >>>>>>>-
> >>>>>>>To unsubscribe from si-list:
> >>>>>>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
> >>>>>>>             =20
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> >>field
> >>   =20
> >>
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> >>>>>>>
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> >>>>>>>
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> >>>>>>>               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
> >>>>>>>
> >>>>>>
> >>>>>>--------------------------------------------------------
> ----------
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> >>>>>>           =20
> >>>>>>
> >>field
> >>   =20
> >>
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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
> >>>>>
> >>>>>* MOVING!  Please note new numbers and address *
> >>>>>
> >>>>>---------------------------------------------------------
> ---------
> >>>>>To unsubscribe from si-list:
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> >>>>>         =20
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> >field
> > =20
> >
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> >>>>
> >>>>------------------------------------------------------------------
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> >>>>       =20
> >>>>
> >>>Doug Brooks' new book, "Signal Integrity Issues and Printed Circuit
> >>>     =20
> >>>
> >Board
> > =20
> >
> >>>Design" has just been released by Prentice Hall. See details and
> >>>     =20
> >>>
> >ordering
> > =20
> >
> >>>info at www.ultracad.com
> >>>
> >>>     =20
> >>>
> >_____________________________________________________________
> __________
> >_____
> > =20
> >
> >>__
> >>   =20
> >>
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> >>
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> >
> >or to administer your membership from a web page, go to:=20
> >//www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >List archives are viewable at:    =20
> >             //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
> --=20
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 2926 SE Yamhill St.
> Portland, OR 97214
> (503) 239-5536
> http://www.teraspeed.com
>=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:=20
> //www.freelists.org/webpage/si-list
>=20
> For help:
>=20
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>=20
> List archives are viewable at:    =20
>               //www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages=20
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>  =20
>=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:
//www.freelists.org/webpage/si-list

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

List archives are viewable at:     
                //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
  

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