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 > >>------------------------------------------------------------------ > > > > > >>To unsubscribe from si-list: > > > > > >>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in=20 > 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=20 > Subject field > > > > > >> > > > > > >>List archives are viewable at: > > > > > >> //www.freelists.org/archives/si-list > > > > > >>or at our remote archives: > > > > > >> =20 > 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=20 > 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=20 > 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 > > > > > > > > > > > > > > > > -- > > > > > Duane Takahashi phone: 408-720-4200 > > > > > Greenfield Networks fax: 408-720-4210 > > > > > 255 Santa Ana Court email:=20 > duanet@xxxxxxxxxxxxxxxxxxxxxx > > > > > Sunnyvale, CA 94085 > > > > > > > > > > * MOVING! Please note new numbers and address * > > > > > > > > > >=20 > ------------------------------------------------------------------ > > > > > To unsubscribe from si-list: > > > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in=20 > 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 > > > > > > > > > > > > > > > > > > > >=20 > >------------------------------------------------------------------ > > > >To unsubscribe from si-list: > > > >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the=20 > 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 > > > > > > > > > > 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 > > > > > > ______________________________________________________________ > ______________ > > __ > > > > > > > > > ------------------------------------------------------------------ > > > To unsubscribe from si-list: > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the=20 > 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: =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 > > > > > > > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the=20 > 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: =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 >=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: > //www.freelists.org/webpage/si-list >=20 > For help: > 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 > ------------------------------------------------------------------ > 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: > //www.freelists.org/webpage/si-list >=20 > For help: > 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