[SI-LIST] Re: general belief about Xtalk
- From: Ihsan Erdin <erdinih@xxxxxxxxx>
- To: Mick zhou <mick.zhou@xxxxxxxxx>
- Date: Fri, 27 Feb 2009 22:39:24 -0500
Before this issue is laid to rest I'd like to add a few comments to TEM mode
of propagation that was mentioned in this thread. The hallmark of TEM is
that the field component in the direction of propagation is zero. In a
linear and isotropic medium, the E and B fields will be perpendicular to
each other regardless of TEM or non-TEM mode of propagation. That condition
may be broken if the medium is nonlinear or anisotropic but this could be
hardly an issue for SI applications. For all practical purposes it is safe
to assume the electric and magnetic fields will remain perpendicular to each
other but there will be field component(s) in the direction of propagation
when TEM is violated at interconnect discontinuities, connectors and such.
Regards,
Ihsan
On Fri, Feb 27, 2009 at 6:36 PM, Mick zhou <mick.zhou@xxxxxxxxx> wrote:
> David,
> Thanks for your inputs and others. There is a similar description in Howard
> Johnson's book, p.204-211. Actually I like Fig.5.16. On. p.207, he even
> states " The forward crosstalk is never larger than the reverse crosstalk".
> It might be the source of the belief. However it cannot be considered as a
> proof at all, and is overtaken. His assumption is "Under normal
> conditions". My point is we should use this statement to judge results
> carefully, especially when extend to other connectors, packages or mixed
> modes.
>
> There are studies (peer-reviewed papers) showing the beliefs are incorrect.
>
> Have a great weekend.
>
> Mick
>
>
> On Mon, Feb 23, 2009 at 5:22 PM, David Banas <david.banas@xxxxxxxxxx>
> wrote:
>
> > The beliefs you give, below, come from the practical observation that,
> > for typical modern PCB designs, capacitive and inductive cross talk have
> > similar magnitudes. Consider:
> >
> > For near end crosstalk:
> > - the capacitive and inductive components reinforce each other, but
> > - the received energy observed at the victim driver is spread out in
> > time,
> > due to the fact that the crosstalk induced energy is traveling in a
> > direction opposite that of the advancing wave in the aggressor.
> >
> > For far end crosstalk:
> > - the capacitive and inductive components cancel each other, but
> > - the received energy observed at the victim receiver is localized in
> > time,
> > because the crosstalk induced energy is traveling along with the
> > advancing
> > aggressor wave.
> >
> > So, to know whether a larger magnitude of crosstalk will be observed at
> > the near or far end requires knowing something about the relative
> > significance of the inductive/capacitive cancelling effects vs. the
> > spreading and localizing effects. In typical modern PCB designs, the
> > inductive and capacitive components of crosstalk cancel rather well and,
> > so, the cancelling ends up being more significant than the spreading,
> > and we get larger magnitude at the near end.
> >
> > -db
> >
> >
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > > On Behalf Of Mick zhou
> > > Sent: Monday, February 23, 2009 3:59 PM
> > > To: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] general belief about Xtalk
> > >
> > > Hi,
> > > Recently, I run into some basic beliefs about Xtalk from my
> > colleagues.
> > > They
> > > seem match intuiations but may not be right.
> > >
> > > Statement 1: Single-ended near-end Xtalk is always greater than
> > far-end
> > > Xtalk, both in f and t domains.
> > > Statement 2: Differential/Common near-end Xtalk is always greater than
> > the
> > > far-end, both in f and t domains.
> > >
> > > I do not remember any general proofs of the above statements. They may
> > be
> > > true for many practical cases but may not be generally true. One
> > example
> > > in
> > > S. H. Hall's book, p.50. disproves the statement 1 in t-domain.
> > >
> > > Any more helps?
> > >
> > > Mick
> > >
> > >
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