Re: [SI-LIST] : return current distribution in diff pairs

Ron Miller (rmiller@Brocade.COM)
Fri, 24 Sep 1999 10:13:47 -0700

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HPeesof ADS also has a color graphical display they use on single and differential
lines which shows the movement of the fields in time(like in a movie).

Ron

"Chan, Michael" wrote:

> Rich:
> Try LC from SGI. It is a FD-TD tool developed by CRAY with help
> from Melinda-Piket-May's group at University of Colorado. But you have
> to have a SGI or CRAY in order to qualify to use the tool.
>
> M.Chan
>
> -----Original Message-----
> From: Mellitz, Richard [mailto:richard.mellitz@intel.com]
> Sent: Thursday, September 23, 1999 4:28 PM
> To: 'si-list@silab.eng.sun.com'
> Subject: RE: [SI-LIST] : return current distribution in diff pairs
>
> Ya know what would be really neat? An FDTD full wave movie showing the two
> wave traveling down the differential pair and then encountering a split on
> the ground reference plane. That way you would see the relative attenuation
> of the wave excites in the spit. Then compare it to the same with single
> ended transmission. Any of you EM gurus out there ever do this? 'Kwon any
> tools that can do this easily?
>
> ...Rich
>
> -----Original Message-----
> From: Eric Bogatin [mailto:eric@bogent.com]
> Sent: Thursday, September 23, 1999 1:17 PM
> To: si-list@silab.eng.sun.com
> Cc: eric
> Subject: [SI-LIST] : return current distribution in
> diff pairs
>
> << File: Current dist.PDF >> Hi guys-
>
> There has been some discussion recently about the current
> distribution of
> the return path in differential pair lines. I think it is a
> common
> misconception that the other line "carries" the return
> current of the first
> line. This may be true when the off diagonal elements of the
> characteristic
> impedance matrix are very small compared to the diagonal
> elements, as in
> shielded twisted pair, but not in typical board geometries.
>
> In the classes I teach, I show an example of the current
> distribution in the
> case of two 50 ohm coupled microstrips, 5 mil line and
> space, coupling of
> about 10%. There is less than 10% overlap of the return
> currents in the
> planes. This is ultimately a "skin depth related" effect. I
> have appended a
> copy of one of my slides showing the current distribution at
> 100 MHz sine
> wave freq for the current in the signal lines and the return
> path in the
> plane below. This was done using the Ansoft Maxwell 2D
> Extractor field
> solver, assuming copper for all the conductors.
> Unfortunately, I can only
> plot the magnitude of the current, not the sign. So, I plot
> in the top
> example, the current in the plane when only one conductor
> has current, +1A,
> showing that most of the return current is directly under
> the signal line.
> Then I plot the current when one has +1A and the other has
> -1A. You can see
> there is clearly a lot of return current in the plane.
>
> The lesson here is to always treat the return currents with
> as much care and
> respect as the signal currents, even in differential pairs,
> unless you know
> for sure the return currents are cancelled in the planes. Of
> course, the
> actual current distribution in the planes will depend on the
> precise cross
> section and spacings.
>
> If the traces go over a split in the return path, the
> currents will probably
> mix, and may go to zero at one spot, but the impedance of
> the two modes will
> be radically changed in this region and you will generate
> common mode
> voltages where there were none before- causing discontinuity
> problems,
> termination problems, switching noise problems and EMI
> problems (did I leave
> any out?). Of course, you need to simulate the magnitude of
> the problem to
> evaluate whether for the given split, the noise is still
> under an acceptable
> limit. But the defensive strategy is treat return paths in
> differential
> pair, like you would for single ended lines.
>
> --eric
>
> Eric Bogatin
> BOGATIN ENTERPRISES
> Training for Signal Integrity and Interconnect Design
> 26235 W. 110th Terr.
> Olathe, KS 66061
> v: 913-393-1305
> f: 913-393-1306
> pager: 888-775-1138
> e: eric@bogent.com
> web: www.bogatinenterprises.com
>
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of D. C.
> Sessions
> > Sent: Wednesday, September 22, 1999 7:35 PM
> > To: si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : Q: Plane-jumping return currents
> >
> >
> > Eric Goodill wrote:
> > >
> > > Mike Jenkins wrote:
> > > >
> > > > Eric,
> > > >
> > > > One line of your question, "My system is running
> pretty fast
> > > > (> 1 Gbps)", caught my eye. At that speed, which I
> assume might
> > > > be Fibre Channel or Gigabit Ethernet, you may well be
> running
> > > > differential. (If not, good luck to you.) But if
> your lines
> > > > are dif'l, they carry their own return current.
> Depending on
> > > > geometry, there is some discontinuity, but MUCH less
> than
> > > > single-ended. If your lines are, in fact,
> differential, and
> > > > if you wish me to elaborate, I will.
> > >
> > > Mike,
> > >
> > > Yes, differential. However, we're using edge-coupled
> pairs, and it's my
> > > understanding, though I've done no analysis, that about
> 10% -
> > 15% is about
> > > as much coupling as you can get between edge-coupled
> lines.
> > Thus, there is
> > > still a strong coupling between the trace and it's
> reference place.
> > > Therefore, I suspect that there's non-ignorable amount
> of
> > return current in
> > > the reference planes. I'd be interested to see a
> > > return-current-distribution plot for a diff pair both in
> the reference
> > > planes and the coupled traces.
> >
> > I don't think so. Sure, there's a fair bit of capacitive
> current between
> > each trace and the adjacent plane, but since they're equal
> and opposite
> > the loop is very small and entirely lateral. Cross a
> plane boundary and
> > there's no need for any current across the break.
> >
> > --
> > D. C. Sessions
> > dc.sessions@vlsi.com
> >
> > **** To unsubscribe from si-list: send e-mail to
> > majordomo@silab.eng.sun.com. In the BODY of message put:
> > UNSUBSCRIBE si-list, for more help, put HELP. si-list
> archives
> > are accessible at http://www.qsl.net/wb6tpu/si-list ****
> >
>
> **** To unsubscribe from si-list: send e-mail to
> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> si-list, for more help, put HELP. si-list archives are accessible at
> http://www.qsl.net/wb6tpu/si-list ****
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

--------------89D4CB158643D9AFE18BECB3 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit

<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> HPeesof ADS also has a color graphical display they use on single and differential
lines which shows the movement of the fields in time(like in a movie).

Ron

"Chan, Michael" wrote:

Rich:
     Try LC from SGI. It is a FD-TD tool developed by CRAY with help
from Melinda-Piket-May's group at University of Colorado. But you have
to have a SGI or CRAY in order to qualify to use the tool.

M.Chan

-----Original Message-----
From: Mellitz, Richard [mailto:richard.mellitz@intel.com]
Sent: Thursday, September 23, 1999 4:28 PM
To: 'si-list@silab.eng.sun.com'
Subject: RE: [SI-LIST] : return current distribution in diff pairs

Ya know what would be really neat?  An FDTD full wave movie showing the two
wave traveling down the differential pair and then encountering a split on
the ground reference plane. That way you would see the relative attenuation
of the wave excites in the spit. Then compare it to the same with single
ended transmission. Any of you EM gurus out there ever do this? 'Kwon any
tools that can do this easily?

...Rich

                -----Original Message-----
                From:   Eric Bogatin [mailto:eric@bogent.com]
                Sent:   Thursday, September 23, 1999 1:17 PM
                To:     si-list@silab.eng.sun.com
                Cc:     eric
                Subject:        [SI-LIST] : return current distribution in
diff pairs

                 << File: Current dist.PDF >> Hi guys-

                There has been some discussion recently about the current
distribution of
                the return path in differential pair lines. I think it is a
common
                misconception that the other line "carries" the return
current of the first
                line. This may be true when the off diagonal elements of the
characteristic
                impedance matrix are very small compared to the diagonal
elements, as in
                shielded twisted pair, but not in typical board geometries.

                In the classes I teach, I show an example of the current
distribution in the
                case of two 50 ohm coupled microstrips, 5 mil line and
space, coupling of
                about 10%. There is less than 10% overlap of the return
currents in the
                planes. This is ultimately a "skin depth related" effect. I
have appended a
                copy of one of my slides showing the current distribution at
100 MHz sine
                wave freq for the current in the signal lines and the return
path in the
                plane below. This was done using the Ansoft Maxwell 2D
Extractor field
                solver, assuming copper for  all the conductors.
Unfortunately, I can only
                plot the magnitude of the current, not the sign. So, I plot
in the top
                example, the current in the plane when only one conductor
has current, +1A,
                showing that most of the return current is directly under
the signal line.
                Then I plot the current when one has +1A and the other has
-1A. You can see
                there is clearly a lot of return current in the plane.

                The lesson here is to always treat the return currents with
as much care and
                respect as the signal currents, even in differential pairs,
unless you know
                for sure the return currents are cancelled in the planes. Of
course, the
                actual current distribution in the planes will depend on the
precise cross
                section and spacings.

                If the traces go over a split in the return path, the
currents will probably
                mix, and may go to zero at one spot, but the impedance of
the two modes will
                be radically changed in this region and you will generate
common mode
                voltages where there were none before- causing discontinuity
problems,
                termination problems, switching noise problems and EMI
problems (did I leave
                any out?). Of course, you need to simulate the magnitude of
the problem to
                evaluate whether for the given split, the noise is still
under an acceptable
                limit. But the defensive strategy is treat return paths in
differential
                pair, like you would for single ended lines.

                --eric

                Eric Bogatin
                BOGATIN ENTERPRISES
                Training for Signal Integrity and Interconnect Design
                26235 W. 110th Terr.
                Olathe, KS  66061
                v: 913-393-1305
                f: 913-393-1306
                pager: 888-775-1138
                e: eric@bogent.com
                web: www.bogatinenterprises.com

                > -----Original Message-----
                > From: owner-si-list@silab.eng.sun.com
                > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of D. C.
Sessions
                > Sent: Wednesday, September 22, 1999 7:35 PM
                > To: si-list@silab.eng.sun.com
                > Subject: Re: [SI-LIST] : Q: Plane-jumping return currents
                >
                >
                > Eric Goodill wrote:
                > >
                > > Mike Jenkins wrote:
                > > >
                > > > Eric,
                > > >
                > > > One line of your question, "My system is running
pretty fast
                > > > (> 1 Gbps)", caught my eye.  At that speed, which I
assume might
                > > > be Fibre Channel or Gigabit Ethernet, you may well be
running
                > > > differential.  (If not, good luck to you.)  But if
your lines
                > > > are dif'l, they carry their own return current.
Depending on
                > > > geometry, there is some discontinuity, but MUCH less
than
                > > > single-ended.  If your lines are, in fact,
differential, and
                > > > if you wish me to elaborate, I will.
                > >
                > > Mike,
                > >
                > > Yes, differential.  However, we're using edge-coupled
pairs, and it's my
                > > understanding, though I've done no analysis, that about
10% -
                > 15% is about
                > > as much coupling as you can get between edge-coupled
lines.
                > Thus, there is
                > > still a strong coupling between the trace and it's
reference place.
                > > Therefore, I suspect that there's non-ignorable amount
of
                > return current in
                > > the reference planes.  I'd be interested to see a
                > > return-current-distribution plot for a diff pair both in
the reference
                > > planes and the coupled traces.
                >
                > I don't think so.  Sure, there's a fair bit of capacitive
current between
                > each trace and the adjacent plane, but since they're equal
and opposite
                > the loop is very small and entirely lateral.  Cross a
plane boundary and
                > there's no need for any current across the break.
                >
                > --
                > D. C. Sessions
                > dc.sessions@vlsi.com
                >
                > **** To unsubscribe from si-list: send e-mail to
                > majordomo@silab.eng.sun.com. In the BODY of message put:
                > UNSUBSCRIBE si-list, for more help, put HELP.  si-list
archives
                > are accessible at http://www.qsl.net/wb6tpu/si-list ****
                >

**** To unsubscribe from si-list: send e-mail to
majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
si-list, for more help, put HELP.  si-list archives are accessible at
http://www.qsl.net/wb6tpu/si-list ****

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

-- 
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017                 
     ==========0000-(_)0000=========== 
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net
  --------------89D4CB158643D9AFE18BECB3-- **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****