> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of tomda
> Sent: Monday, September 27, 1999 8:51 AM
> To: '[email protected]'
> Subject: RE: [SI-LIST] : return current distribution in diff pairs
>
>
> Does this assume that the two wave fronts are exactly time coincident?
> What happens if real world skew is introduced? What happens if the rise
> and fall times are not symetrical?
>
> Tom Dagostino
>
> -----Original Message-----
> From: Chris Cheng [SMTP:[email protected]]
> Sent: Friday, September 24, 1999 3:01 PM
> To: [email protected]
> Subject: RE: [SI-LIST] : return current distribution in diff pairs
>
> .
> .
> .
> .
> .
> .
> no need to do fancy fttd. just use a simple free tool like fasthenry
> from mit (in fact i think its in one of its example)
> and u will notice when differential signals jump reference
> plane, the image currents of the signals overlap each other and
> travel along the edge of the plane and nearly cancel each other
> out. the differential return loop is very small across the
> boundary since the both current is force to travel with each other
> (i.e. strong coupling and cancellation). i don't think differential
> signals crossing boundary is a significant problem.
> chris
> >
> > > -----Original Message-----
> > > From: [email protected]
> > > [mailto:[email protected]]On Behalf Of Mellitz, Richard
> > > Sent: Thursday, September 23, 1999 2:28 PM
> > > To: '[email protected]'
> > > 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:[email protected]]
> > > Sent: Thursday, September 23, 1999 1:17 PM
> > > To: [email protected]
> > > 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: [email protected]
> > > web: www.bogatinenterprises.com
> > >
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: [email protected]
> > > > [mailto:[email protected]]On Behalf Of D. C.
> > > Sessions
> > > > Sent: Wednesday, September 22, 1999 7:35 PM
> > > > To: [email protected]
> > > > 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
> > > > [email protected]
> > > >
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> > >
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