[SI-LIST] Re: Diff.Pairs

What you say is probably true,
but it is the difference significant? Is it an order of magnitude?
I believe that a huge portion of the return current is still in the ground
plane and so relying on differential signals to significantly effect the
crossing of
a ground split is not a reasonable option.


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Knighten, Jim L
Sent: Thursday, October 09, 2003 5:09 PM
To: richard.mellitz@xxxxxxxxx; weirsp@xxxxxxxxxx;
leeritchey@xxxxxxxxxxxxx; Doug Brooks; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs


Richard,

<Closely coupled lines traverse plane splits and other CM producing
layout structures better than uncoupled lines.>

This is consistent with the fact that closely (tightly) coupled lines are
coupled more strongly to one another than to the reference plane.  There is
less differential mode current (the circulating currents that net out to
zero) in the reference plane than for the case of loosely coupled lines.
Less current on the plane means less disturbance when you cross a
discontinuity in the plane.

... 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: Mellitz, Richard [mailto:richard.mellitz@xxxxxxxxx]
Sent: Thursday, October 09, 2003 3:42 PM
To: weirsp@xxxxxxxxxx; Knighten, Jim L; leeritchey@xxxxxxxxxxxxx; Doug
Brooks; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs

Above a GHz, delta D+ and D- results in CM which can cause the eye to
close because of accumulation of charge on the lines. I.e. D+ goes one
way and D- goes the other. It basically has the same effect as duty
cycle distortion. Also some busses spec max AC CM.=20

Closely coupled lines traverse plane splits and other CM producing
layout structures better than uncoupled lines.=20

Closely coupling lines does not guarantee the electrical characteristics
for each line are exactly the same.=20

If you bus has lots of margin, who cases what you do. If not, you'll
need to do some engineering to split the hairs and will need
comprehensive electrical models that are validated against measurements.
At that point its cost function and the trade offs are yours.

... Rich

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Thursday, October 09, 2003 2:36 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
> > > > >>
> > > > >>
> > > > >>
> > > >
>>------------------------------------------------------------------
> > > > >>To unsubscribe from si-list:
> > > > >>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the
Subject
>field
> > > > >>
> > > > >>or to administer your membership from a web page, go to:
> > > > >>http://www.freelists.org/webpage/si-list
> > > > >>
> > > > >>For help:
> > > > >>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > > > >>
> > > > >>List archives are viewable at:
> > > > >>                http://www.freelists.org/archives/si-list
> > > > >>or at our remote archives:
> > > > >>                http://groups.yahoo.com/group/si-list/messages
> > > > >>Old (prior to June 6, 2001) list archives are viewable at:
> > > > >>                http://www.qsl.net/wb6tpu
> > > > >>
> > > > >
> > > > >
> > > > >
> > > > >
------------------------------------------------------------------
> > > > > To unsubscribe from si-list:
> > > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the
Subject
>field
> > > > >
> > > > > or to administer your membership from a web page, go to:
> > > > > http://www.freelists.org/webpage/si-list
> > > > >
> > > > > For help:
> > > > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > > > >
> > > > > List archives are viewable at:
> > > > >             http://www.freelists.org/archives/si-list
> > > > > or at our remote archives:
> > > > >             http://groups.yahoo.com/group/si-list/messages
> > > > > Old (prior to June 6, 2001) list archives are viewable at:
> > > > >             http://www.qsl.net/wb6tpu
> > > > >
> > > >
> > > > --
> > > > 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 *
> > > >
> > > >
------------------------------------------------------------------
> > > > To unsubscribe from si-list:
> > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
field
> > > >
> > > > or to administer your membership from a web page, go to:
> > > > http://www.freelists.org/webpage/si-list
> > > >
> > > > For help:
> > > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > > >
> > > > List archives are viewable at:
> > > >               http://www.freelists.org/archives/si-list
> > > > or at our remote archives:
> > > >               http://groups.yahoo.com/group/si-list/messages
> > > > Old (prior to June 6, 2001) list archives are viewable at:
> > > >               http://www.qsl.net/wb6tpu
> > > >
> > >
> > >
> > >
> > >------------------------------------------------------------------
> > >To unsubscribe from si-list:
> > >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
field
> > >
> > >or to administer your membership from a web page, go to:
> > >http://www.freelists.org/webpage/si-list
> > >
> > >For help:
> > >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > >List archives are viewable at:
> > >                 http://www.freelists.org/archives/si-list
> > >or at our remote archives:
> > >                 http://groups.yahoo.com/group/si-list/messages
> > >Old (prior to June 6, 2001) list archives are viewable at:
> > >                 http://www.qsl.net/wb6tpu
> > >
> >
> > 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
> >
>_______________________________________________________________________
_____
>__
> >
> >
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
field
> >
> > or to administer your membership from a web page, go to:
> > http://www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List archives are viewable at:
> >               http://www.freelists.org/archives/si-list
> > or at our remote archives:
> >               http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> >               http://www.qsl.net/wb6tpu
> >
>
>
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>http://www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List archives are viewable at:
>                 http://www.freelists.org/archives/si-list
>or at our remote archives:
>                 http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
>                 http://www.qsl.net/wb6tpu
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>http://www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List archives are viewable at:
>                 http://www.freelists.org/archives/si-list
>or at our remote archives:
>                 http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
>                 http://www.qsl.net/wb6tpu
>


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:    =20
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages=20
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
 =20

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: