Steve: My understanding the use of closely coupled differential pair is = to try to reduce the amount of net common-mode current on the power/ground = reference planes for=20 reduction of common-mode radiation in EMI. Ideally, if a pair of = differential signals ( with no skew ) are tigtly coupled and close to reference plane, their image currents = impressed onto the plane=20 will be effectively cancelled and net common-mode current can be = minimized and so do common-mode radiation. However, in the real world this is almost next to impossible = to achieve and the result might not be that great as thought. Just my 2 cents. Regards, Michael -----Original Message----- From: steve weir [mailto:weirsp@xxxxxxxxxx] Sent: Thursday, October 09, 2003 1:36 PM To: JK100005@xxxxxxxxxxxxxxxx; leeritchey@xxxxxxxxxxxxx; Doug Brooks; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Diff.Pairs Jim, Given that any PWB will have lots of longitudinal current, I think the = net=20 CM current of a pair is way down the list of effects that we should be=20 concerned with. I have to agree with the notion that the main = motivation=20 of a diff pair on a board is reliable signaling, and not current = balance. It seems to me the primary motivation for keeping the two halves of a = pair=20 together is that material variations then affect the flight time of both = about the same. With that in mind, I am more interested in comments and = techniques of maintaining electrical length matching within tolerable=20 limits with disparate routing of the two signals considering = manufacturing=20 and material variations in the PWB itself. Regards, Steve. At 01:40 PM 10/9/2003 -0400, Knighten, Jim L wrote: >Lee, > >Your post is interesting! > >Differential signaling is usually implemented with coupled transmission >lines. The mutual coupling between the traces affect the two modes = that are >always present (even and odd modes). In the traditional configuration, = the >two traces are parallel and of the same width and thickness and located >adjacent to a plane. The degree of coupling between the traces is = usually >described as "loosely coupled" or "tightly coupled." In either case, = if the >signal and signal traces are perfectly differential (i.e., no = imbalance, >perfectly symmetrical), then there is always current in the adjacent = ground >plane, but the net current in the longitudinal direction (the direction = of >the traces) is zero. The currents that exist in the adjacent plane are >circulating currents that reflect the distributed coupling between the >traces down the length of the transmission line. > >So, what if the two coupled traces are not co-planar, i.e., not in the = same >plane? Well, you still have two coupled transmission lines, but the = mutual >capacitance and inductance between them may be different than if they = were >co-planar, hence the even and odd mode impedances may be different. = These >non-co-planar coupled lines can still carry differential signals, = though. > >What if the two coupled lines were not co-planar and actually had the = ground >plane between them? This is just a special case of the "loosely = coupled" >case, in that the lines are now not coupled at all. Still, the lines = can >support differential signaling, but the relationships between even and = odd >modes are not quite the same as when they were coupled. (Perhaps even = mode >and odd mode impedances are equal?) > >So, how about current in the ground plane? For perfect differential >signaling, the net current in the plane is zero. When you introduce >imbalance, either in the signal source, or in the signal path, you = create >net longitudinal current in the ground plane. This is the even mode = signal, >which has no bearing on your intended differential signal (the odd = mode) and >represents an EMI source on the ground plane. > >If you route differential signals on different layers, it may be more >difficult to maintain balance (symmetry) in the traces than if the = traces >were co-planar. If this is true, you have more potential for 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 = planes >of a PCB, the return current for each member of the pair travels on the >plane over which it travels, not on the other wire. If they are very >tightly coupled to each other, perhaps 5% of the current from one = travels >in the other. It is coincidental that the two currents are equal in >magnitude and opposite. They don't have to be. Their "return = currents" >still travel on the plane, not on the other wire. > >As far as EMI is concerned, it has been demonstrated many times, once = in >the paper done by Doug Brooks with the staff at UMR, that traces = traveling >over planes are not a detectable source of EMI. Therefore, = constraining >the routing of differential pairs to prevent them from creating EMI is = not >appropriate or necessary. > >It is still true that the two members of a differential pair are two >independent signals traveling on two independent transmission lines. = All >they have in common is that the have equal amplitudes and are 180 = degrees >out of phase with each other. If the protocol is LVDS, each member of = the >pair should be parallel terminated in an impedance equal to Zo for that >line to Vref (about 1.25V) which is half way between the two logic = levels. > >As long as the two signals switch at the same time, the current flowing = out >of Vref into one line is the same magnitude an opposite in polarity to = that >flowing into the other. The net current into and out of the Vref = terminal >is zero, so we can omit the connection. When we do this, we have two >resistors, each of value Zo across the ends of the two transmission = lines. >For convenience, we use one resistor of value 2 X Zo. This is not a >differential impedance of 100 ohms, but two parallel terminations of = value >Zo terminating two transmission lines each of impedance Zo. > >As long as the two edges switch at the same time, there is no current >imbalance and all is well. Soon as one edge switches before the other, >there is a need for a momentary current spike to flow into or out of = the >Vref terminal. If there is no connection to Vref for the current flow, = the >result is the edges are degraded. To avoid this degradation, a very = small >capacitor is often connected between the two resistors and ground. = This is >a very common termination for 2.4 GB/S signal links. > >It is time to stop representing differential signals as needing to be >tightly coupled to each other in order to operate properly. It is = simply >not so. I have routed thousands of differential signal where each = member >of the pair is on a different layer. If this were not possible, 1 mm = pitch >BGAs with differential signals would be un routable. There are tens of >thousands of such parts being shipped every month on PCBs where they = are >routed apart from each other. > >This is all described in my recently published book, "Right the First = Time, >A Practical Handbook on High Speed PCB and System Design". It is also >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 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 > > > > > > > > > > = =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 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 > > > > >> > > > > >> > > > > >> > > > > = >>------------------------------------------------------------------ > > > > >>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 > > > > >> > > > > > > > > > > > > > > > > > > > > = ------------------------------------------------------------------ > > > > > 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 > > > > > > > > > > > > > -- > > > > 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: > > > > 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 > > > > > > > > > > > > > > > >------------------------------------------------------------------ > > >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 > > > > > > > Doug Brooks' new book, "Signal Integrity Issues and Printed Circuit = Board > > Design" has just been released by Prentice Hall. See details and = ordering > > info at www.ultracad.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: > > //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 > > > > > >------------------------------------------------------------------ >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 > >------------------------------------------------------------------ >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 > ------------------------------------------------------------------ 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: =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 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