[SI-LIST] Re: Diff.Pairs

The common mode rejection needs to be considered in some cases.  There =
is a trade-off.

Mick

-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
Sent: Thursday, October 09, 2003 1:57 PM
To: Knighten, Jim L; Doug Brooks; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs


Why do people try so hard to make the coupling between the two members =
of a
pair so important.  These are two independent signals that travel over
planes independently unless they are placed close enough together that
there is some interaction.  This interaction is not beneficial.

Lee


> [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 =
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. =20
>
> 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]=20
> 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.=20
> 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.=20
>
> 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"=20
> > in EMI terms. That is, the loop is the area encompassed by the =
signal
and=20
> > its return. Smaller loop areas perform better than larger loop areas
when=20
> > EMI is a concern. The closer the differential pair, the smaller is =
the=20
> > loop. If we are NOT concerned about EMI, then this is not an issue. =
If
we=20
> > ARE, then we might want to pay attention to this and keep the loop =
small
> by=20
> > routing the traces close together.
> >
> > The equal spacing "requirement" comes from the control of =
reflections
(ie=20
> > transmission line termination issues.) IF we are concerned about=20
> > reflections, THEN we need a constant impedance everywhere along the
> trace.=20
> > IF the (differential) traces are close together (for EMI reasons) =
THEN
> they=20
> > will interact (a very special case of crosstalk, which in this
particular=20
> > case [signals --- being equal and opposite --- are exactly =
correlated
> with=20
> > each other] is not a problem.) IF we want to keep a constant =
impedance=20
> > along the traces, THEN we must keep a "constant" spacing 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 being that =
the=20
> > 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=20
> > signals is that they are equal and opposite, and therefore there is =
no=20
> > return signal through the ground system. Even if the signals are
perfect,=20
> > if the traces are different length, then the signal will not arrive =
at
> the=20
> > far end at exactly the same time and the signals will not be "equal =
and=20
> > opposite" at the receiver. Just a couple of degrees phase shift can =
make
> a=20
> > surprising difference between the signals when we are talking about=20
> > (square-wave) clock signals. If the signals are not exactly equal =
and=20
> > opposite, then there MUST be a net current flowing somewhere else. =
This=20
> > 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=20
> > themselves. This is important to recognize. But if are concerned =
about=20
> > certain SI issues, they might lead to some design considerations =
which
> THEN=20
> > 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=20
> > Design" has just been released by Prentice Hall. See details and
ordering=20
> > info at www.ultracad.com
> >
>
_________________________________________________________________________=
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