[SI-LIST] Re: Diff.Pairs
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Dunbar, Tony" <tony_dunbar@xxxxxxxxxxx>,"si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 9 Oct 2003 18:04:11 -0700
IC designers should pin out pairs close together in order to make it easier
to maintain length matching between members of the pair. Otherwise, it
doesn't matter.
Lee
> [Original Message]
> From: Dunbar, Tony <tony_dunbar@xxxxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx <si-list@xxxxxxxxxxxxx>
> Date: 10/9/2003 4:01:42 PM
> Subject: [SI-LIST] Re: Diff.Pairs
>
> Lee,
>
> Should we, then, not be concerned if the Si or package vendor chooses to
> pin-out the device with differential signal pairs all over the package,
> rather than together?
>
> Regards,
> Tony
>
> -----Original Message-----
> From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> Sent: Thursday, October 09, 2003 12:57 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.
> >
> > 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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> > >
> > > 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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