[SI-LIST] Re: Even mode, common mode, and mode conversion

  • From: Aubrey_Sparkman@xxxxxxxx
  • To: sainath@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Tue, 26 Feb 2002 15:04:29 -0600

Sainath,
Perhaps I can help.  Seems like there are two different types of modes being
confused here.

Type 1)
Any single transmission line will propagate the TEM mode up to it's cut-off
frequency, then you get other modes like TE11, TM11, etc which means one of
the cross-sectional dimensions is greater than 1/2 wavelength (if memory
serves, microwave experts feel free to correct me) but you should
investigate waveguides if you want to understand all these different types
of modes.  I haven't seen this type of modes be relevant to SI engineers in
the past.  Perhaps the hi speed serial links will change that.  Most of our
chip-to-chip signals (on the same PCB) signal each other using TEM mode.

Type 2)
SI engineers talk about odd and even modes when a pair of signals are either
in phase or 180 degrees out of phase. The pair of odd mode signals see a
differential impedance and SI engineers call that differential signaling.
The noise that disturbs the pair is usually "in phase" and we hope for good
common mode rejection; the differential receiver sees no difference.  If you
consider the odd mode pair having an "a" signal and a "b" signal, the "a"
signal will see the odd mode impedance when the pair of signals are 180
degrees out of phase.  The "a" signal will see the even mode impedance when
the pair of signals are in phase.

Hope this helps.

Aubrey Sparkman 
Signal Integrity 
Aubrey_Sparkman@xxxxxxxx 
(512) 723-3592



-----Original Message-----
From: sainath@xxxxxxxxxxxxx [mailto:sainath@xxxxxxxxxxxxx]
Sent: Tuesday, February 26, 2002 1:31 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion



Steve,

I better leave my three questions at that with all those 'refer to's.
My attention is turned to the statement: "A system doesn't really support
modes, it has modes."
Again, is it Paul's book that I should refer to?

Thanks,
Sainath

----- Original Message -----
From: "Steve Corey" <steve@xxxxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, February 26, 2002 10:48 AM
Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion


>
> Sainath -- I'll see what I can do without getting too embroiled in the
> details:
>
> 1. What makes the two T-lines support even or odd kind of modes? and the
> one T-line support the TEM kind of modes?
>
> For the first part of #1, I would refer to the book by Paul.  A system
> doesn't really support modes, it has modes.  Sometimes in analysis we
> break down voltages and currents into the system modes to simplify
> analysis.  If you come up with a good answer to the first part of #1,
> you will understand that the even and odd are also TEM modes in the same
> way that a single line has a TEM mode.
>
> 2.  Is it not possible to have TEM kind of mode propagation in a two
> T-line system?
>
> Yes, as you will see in my previous post.
>
> 3.  How would you define a MODE when it is meant even or odd mode ? and
> when it is meant TEM mode?
>
> For the first part of #3, you can refer to Paul's book for a rigorous
> mathematical definition.  You can also refer to Eric Bogatin's post to
> this thread for what may be a more intuitive definition.  For the second
> part of the question, refer to my earlier post for a short definition,
> or to Paul's book or any E/M book that covers waveguides for a longhand
> definition.
>
>
>  > >
>  > >
>
>
>
> Sainath Nimmagadda wrote:
>
> > Steve,
> >
> > I suppose your message was to address my questions to Arpad.
> > There are 3 questions and I'm not clear which one is addressed.
> > Is it possible to say few simple words about each question separately?
> >
> > Thanks,
> > Sainath
> >
> > ----- Original Message -----
> > From: "Steve Corey" <steve@xxxxxxxxxxxxxx>
> > To: <si-list@xxxxxxxxxxxxx>
> > Sent: Tuesday, February 26, 2002 7:20 AM
> > Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion
> >
> >
> >
> >>Sainath -- I will give you the short answer, and refer you to the texts
> >>for the long answer.  The even and odd modes are TEM modes, or at least
> >>quasi-TEM.  TEM vs. higher order modes describes the field distribution
> >>across a single transmission line or waveguide structure.  Quasi-TEM is
> >>a basic assumption of the transmission line theory on which we rely,
> >>including characteristic impedance and RLGC matrices.  It requires that
> >>to first order, no field has a component in the direction of
> >>propagation.  I usually rely on "Analysis of Multiconductor Transmission
> >>Lines" by Clayton R. Paul when I have questions of this sort.
> >>
> >>The higher order modes are significant to transmission line theory for
> >>the most part in that we try to make sure they don't exist, since they
> >>each propagate with their own velocities and make a mess of our nice
> >>clean square waves.  Any E/M text with a waveguide section will describe
> >>the physics behind this.
> >>
> >>On the other hand, all TEM modes propagate.  Analysis of the different
> >>TEM modes supported on a multiconductor transmission line structure is
> >>from my perspective more of a mathematical tool for breaking down a
> >>system into its most basic components.  In the case of even/odd, this
> >>has a very practical application in differential signaling, but once we
> >>step outside of the symmetric coupled two-line regime, the math gets
> >>pretty complicated to try and define signaling schemes which coincide
> >>with any particular modes.
> >>
> >>So-called "full-wave" analysis, which essentially means "non-TEM
> >>analysis" for our purposes, is necessary for situations where we can't
> >>count on quasi-TEM assumptions.  These situations occur when we step
> >>outside of the strict geometries of uniform transmission lines, or into
> >>certain loss regimes.  Again, I would refer you to the book by Clayton
> >>R. Paul.
> >>
> >>   -- Steve
> >>
> >>-------------------------------------------
> >>Steven D. Corey, Ph.D.
> >>Time Domain Analysis Systems, Inc.
> >>"The Interconnect Modeling Company."
> >>http://www.tdasystems.com
> >>
> >>email: steve@xxxxxxxxxxxxxx
> >>phone: (503) 246-2272
> >>fax:   (503) 246-2282
> >>-------------------------------------------
> >>
> >>
> >>Muranyi, Arpad wrote:
> >>
> >>
> >>>To all,
> >>>
> >>>Sainath addressed his question to me, but I am going on vacation
> >>>and don't have the time to answer it.  Could someone please
> >>>give him a good explanation to his questions?  Thanks,
> >>>
> >>>Arpad
> >>>=================================================================
> >>>
> >>>-----Original Message-----
> >>>From: sainath@xxxxxxxxxxxxx [mailto:sainath@xxxxxxxxxxxxx]
> >>>Sent: Monday, February 25, 2002 11:18 AM
> >>>To: si-list@xxxxxxxxxxxxx
> >>>Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion
> >>>
> >>>
> >>>
> >>>Arpad,
> >>>
> >>>This two T-lines vs one T-line mode distinction is confusing to me.
> >>>What makes the two T-lines support even or odd kind of modes? and the
> >>>
> > one
> >
> >>>T-line support the TEM kind of modes?
> >>>Is it not possible to have TEM kind of mode propagation in a two T-line
> >>>system?
> >>>How would you define a MODE when it is meant even or odd mode ? and
when
> >>>
> > it
> >
> >>>is meant TEM mode?
> >>>
> >>>Thanks,
> >>>Sainath
> >>>
> >>>----- Original Message -----
> >>>From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx>
> >>>To: <si-list@xxxxxxxxxxxxx>
> >>>Sent: Monday, February 25, 2002 10:26 AM
> >>>Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion
> >>>
> >>>
> >>>
> >>>
> >>>>What may be confusing to some is that the word "mode" is used
> >>>>in different ways in different contexts, both involving
> >>>>transmission lines.
> >>>>
> >>>>So far most responses mentioned the even or odd mode, where
> >>>>the signals of ***two*** T-lines go either in the same or the
> >>>>opposite direction.
> >>>>
> >>>>The other usage of this word involves only one T-line (or
> >>>>wave guide or optical fiber) and describes how the electro
> >>>>magnetic waves propagate inside that line, one of the most
> >>>>familiar mode being the TEM (Transverse Electro Magnetic)
> >>>>mode.  Don't confuse the two meanings of the word MODE!
> >>>>
> >>>>Arpad Muranyi
> >>>>Intel Corporation
> >>>>=================================================================
> >>>>
> >>>>
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> --
> -------------------------------------------
> Steven D. Corey, Ph.D.
> Time Domain Analysis Systems, Inc.
> "The Interconnect Modeling Company."
> http://www.tdasystems.com
>
> email: steve@xxxxxxxxxxxxxx
> phone: (503) 246-2272
> fax:   (503) 246-2282
> -------------------------------------------
>
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