[SI-LIST] Re: Diff.Pairs
- From: "Chris McGrath" <chris.mcgrath@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 9 Oct 2003 13:27:12 -0700
That having been said, are there any ramifications if ther differential
driver is balanced vs. unbalanced? =20
Thanks,
Chris
> -----Original Message-----
> From: Dunbar, Tony [mailto:tony_dunbar@xxxxxxxxxxx]=20
> Sent: Thursday, October 09, 2003 2:56 PM
> To: 'si-list@xxxxxxxxxxxxx'
> Subject: [SI-LIST] Re: Diff.Pairs
>=20
>=20
> Lee,
>=20
> Should we, then, not be concerned if the Si or package vendor=20
> chooses to pin-out the device with differential signal pairs=20
> all over the package, rather than together?
>=20
> Regards,
> Tony
>=20
> -----Original Message-----
> From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> Sent: Thursday, October 09, 2003 12:57 PM
> To: Knighten, Jim L; Doug Brooks; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Diff.Pairs
>=20
>=20
> Why do people try so hard to make the coupling between the=20
> two members of a pair so important. These are two=20
> independent signals that travel over planes independently=20
> unless they are placed close enough together that there is=20
> some interaction. This interaction is not beneficial.
>=20
> Lee
>=20
>=20
> > [Original Message]
> > From: Knighten, Jim L <JK100005@xxxxxxxxxxxxxxxx>
> > To: <leeritchey@xxxxxxxxxxxxx>; Doug Brooks <doug@xxxxxxxxxx>;
> <si-list@xxxxxxxxxxxxx>
> > 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
> are
> > 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
> the
> > signal and signal traces are perfectly differential (i.e., no=20
> > imbalance, perfectly symmetrical), then there is always=20
> current in the=20
> > adjacent
> ground
> > 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
> same
> > plane? Well, you still have two coupled transmission lines, but the
> mutual
> > 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
> >
> > What if the two coupled lines were not co-planar and=20
> actually had the
> ground
> > 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
> signal,
> > which has no bearing on your intended differential signal (the odd=20
> > mode)
> and
> > 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
> out
> > of Vref into one line is the same magnitude an opposite in=20
> polarity to
> that
> > 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
> 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,
> the
> > 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
> is
> > 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
> pitch
> > 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
> Time,
> > 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
> moment..
> >
> > Lee
> >
> >
> > > [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=20
> constitutes a
> > "loop"
> > > in EMI terms. That is, the loop is the area encompassed by the=20
> > > signal
> and=20
> > > its return. Smaller loop areas perform better than larger=20
> loop areas
> when=20
> > > EMI is a concern. The closer the differential pair, the=20
> smaller is=20
> > > the
> > > loop. If we are NOT concerned about EMI, then this is not=20
> an issue. If
> we=20
> > > ARE, then we might want to pay attention to this and keep=20
> the loop=20
> > > small
> > by
> > > routing the traces close together.
> > >
> > > The equal spacing "requirement" comes from the control of=20
> > > reflections
> (ie=20
> > > transmission line termination issues.) IF we are concerned about
> > > reflections, THEN we need a constant impedance everywhere=20
> along the
> > trace.
> > > IF the (differential) traces are close together (for EMI reasons)=20
> > > THEN
> > they
> > > will interact (a very special case of crosstalk, which in this
> particular=20
> > > case [signals --- being equal and opposite --- are exactly=20
> > > correlated
> > with
> > > 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=20
> being that=20
> > > the
> > > differential traces must be equal length. This is NOT for timing
> reasons,=20
> > > but for common mode reasons. A strong assumption we make about
> > differential
> > > signals is that they are equal and opposite, and=20
> therefore there is=20
> > > no
> > > return signal through the ground system. Even if the signals are
> perfect,=20
> > > if the traces are different length, then the signal will=20
> not arrive=20
> > > at
> > the
> > > 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
> > a
> > > surprising difference between the signals when we are=20
> 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
> > issue.
> > >
> > > None of the differential signal trace design rules are necessary=20
> > > taken
> 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
> > 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=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=20
> > > >in good
> old
> > > >cross talk that actually degrades the edges.
> > > >
> > > >How the notion of tight coupling of differential pairs as=20
> > > >beneficial
> got
> > > >started is a mystery to me. There are several=20
> 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=20
> released=20
> > > >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=20
> calculations
> when
> > > > > > using woven glass reinforced=20
> > > > > >=20
> <http://www.polarinstruments.com/support/cits/AP139.html>lamin
> > > > > > ates
> > > > > >
> > > > > > 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
> > > > > > =
=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:
> > > > > >
> > > > > >>I am seeking help in finding enlightenment=20
> regarding electrical
> > > > > >>performance pros and cons and how manufacturing=20
> tolerances play a
> > role
> > > > > >>when comparing side by side and tandem differential=20
> 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
> > > > > >>
> > > > > >>
> > > > > >>
> > > > >=20
> >>------------------------------------------------------------------
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> > > > >
> > > > > --
> > > > > Duane Takahashi phone: 408-720-4200
> > > > > Greenfield Networks fax: 408-720-4210
> > > > > 255 Santa Ana Court email:=20
> duanet@xxxxxxxxxxxxxxxxxxxxxx
> > > > > Sunnyvale, CA 94085
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> > > > >
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> > >
> > > Doug Brooks' new book, "Signal Integrity Issues and=20
> Printed Circuit
> Board=20
> > > Design" has just been released by Prentice Hall. See details and
> ordering=20
> > > info at www.ultracad.com
> > >
> >
> ______________________________________________________________
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