[SI-LIST] Re: general belief about Xtalk
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: "'Ihsan Erdin'" <erdinih@xxxxxxxxx>, "'Mick zhou'" <mick.zhou@xxxxxxxxx>
- Date: Sat, 28 Feb 2009 08:17:03 -0800
I agree with Ihsan in general, but would like to share some thoughts on the
TEM waves.
The waves in interconnects can be considered as TEM (no E and H along the
lines) only in cases of strip-line structures with homogeneous dielectric
and with lossless conductor. Even lossless micro-strip line has quasi-TEM
dominant mode with both E and H components along the propagation direction
due to the boundaries between dielectrics (the effect of these components
can be observed above about 5 GHz for typical PCB interconnects). In case of
lossy conductors, both strip and micro-strip lines have longitudinal
components of E and H that are growing with the frequency. Losses and
electromagnetic energy absorption by conductor generates electric field
along the propagation direction simply due to the Ohm's low E=r*j - this
violates the TEM nature of the fields in interconnects with lossy
conductors. Belief in TEM mode is probably based on the common use of static
field solvers for analysis of such multi-conductor interconnects. Though,
the static solution is typically just the starting point in such analysis.
Losses in conductors and dielectrics are added as post-processing of the
static solution (with perturbation analysis and analytical formulas for
conductor loss and with solution with complex dielectric constant at one
frequency for dielectric loss). It provides good approximation even for
non-TEM waves in cases if some additional conditions are satisfied - such as
the cross-section must be much smaller than wavelength at the highest
frequency of interest, smallness of conductor-related losses, identical loss
and dispersion in all dielectrics and so on - the detailed analytical
analysis is provided in F. Olyslager, Electromagnetic waveguides and
transmission lines, Oxford, 1999, p.29.
Note, that being non-TEM or quasi-TEM, the waves in multi-conductor lines
remain transverse even at very high frequencies (the phase of the fields
depends only on the position along the line) and Telegrapher's equations can
be still used with appropriate impedance and admittance per unit length
extracted with the full-wave (non-static) analysis. On the other hand, the
fields near the discontinuity are not transverse in general - that may be
distinguishing property of a discontinuity. Though, the fields can be still
expanded into a set of the transverse waves (both propagating and
evanescent) in any segment of a line close to the discontinuity - this is
just analogue of the Fourier expansion.
Best regards,
Yuriy Shlepnev
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Ihsan Erdin
Sent: Friday, February 27, 2009 7:39 PM
To: Mick zhou
Cc: David Banas; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: general belief about Xtalk
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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