[SI-LIST] Re: Diff.Pairs

  • From: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxx>
  • To: leeritchey@xxxxxxxxxxxxx, Doug Brooks <doug@xxxxxxxxxx>,si-list@xxxxxxxxxxxxx
  • Date: Thu, 9 Oct 2003 13:40:24 -0400

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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> > >
> > > --
> > > 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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> >
> >
> >
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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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