[SI-LIST] Re: Diff.Pairs

  • From: "Chan, Michael (Eng Hou)" <michael.chan@xxxxxx>
  • To: <weirsp@xxxxxxxxxx>, <JK100005@xxxxxxxxxxxxxxxx>,<leeritchey@xxxxxxxxxxxxx>, "Doug Brooks" <doug@xxxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
  • Date: Thu, 9 Oct 2003 14:56:56 -0500

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
> > > > >>
> > > > >>
> > > > >>
> > > > =
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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
> > > >
> > > > * MOVING!  Please note new numbers and address *
> > > >
> > > > =
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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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