[SI-LIST] Re: Diff.Pairs

I always thought that an additional advantage of differential signaling
(with a differential receiver) was common mode noise rejection at the
receiver.
The closer the lines are, for this example, the more the noise would be
symmetric and the
better the rejection.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Thursday, October 09, 2003 11:36 AM
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
CM current of a pair is way down the list of effects that we should be
concerned with.  I have to agree with the notion that the main motivation
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
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
limits with disparate routing of the two signals considering manufacturing
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
> > > > >
> > > > > ============================================
> > > > >   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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> >
> > 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
> >
>___________________________________________________________________________
_
>__
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