[SI-LIST] Re: Discontinuities on PCB

  • From: Ronald.De_Smedt@xxxxxxxxxx
  • To: Charles Harrington <ch_harrington@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Thu, 12 May 2005 21:55:14 +0200

Hello Charles,

I assume that by the 'point of excitation' you mean the location
of the discontinuity. Higher order modes, which are well below
cut-off, typically go only a few times a typical dimension of
the cross section. In the case of microstrip, this means they
exist up to a distance of one or two times the width of the line.
This is true for the first few higher order modes. When the order
increases, the attenuation increases as well and, hence, this
distance to the discontinuity will become smaller as well.
Of course, when the frequency approaches the cut-off frequency
of a next order mode, this mode will be less attenuated and
will extend to a larger distance.

Hopefully, this answers your question.
Best greetings,

Dr. Ronald De Smedt
Alcatel Bell, F. Wellesplein 1, B-2018 Antwerpen (Belgium)

Charles Harrington wrote:
> 
> Thanks very much Ronald!
> Your explanation of the phenomenon is excellent, but
> do you have any idea how one can estimate how far away
> from the point of excitation the modes below the
> cut-off frequency (non-propagating) can go?
> Thanks
> Charles
> 
> --- Ronald.De_Smedt@xxxxxxxxxx wrote:
> > Hello,
> >
> > Besides of the good advice to consult some excellent
> > books,
> > here is an intuitive explanation of the phenomenon:
> >
> > The situation is very similar to what happens in a
> > waveguide with
> > discontinuities (although the modes are different,
> > as they depend
> > on the cross section). As long as the actual cross
> > section matches
> > the cross section for which the mode has been
> > defined, it just
> > propagates (and/or attenuates) in the longitudinal
> > direction
> > without any change in the transverse direction (=
> > cross section).
> > However, when it hits a discontinuity, the fields of
> > the mode
> > don't match any more with the actual environment. As
> > a result
> > a scattered field is excited such that the total
> > field (original
> > incident mode + scattered field) do satisfy all
> > necessary
> > boundary and continuity conditions. Outside of the
> > discontinuity
> > region, the scattered fields can be decomposed into
> > the various
> > modes of the 'launching' lines. One of these modes
> > will be the
> > lowest (quasi-)TEM mode (in the microstrip), the
> > others are
> > the higher-order modes (similar to what happens in
> > waveguides).
> > In case of the microstrip, which consists of a mixed
> > environment
> > (dielectric + air), these higher modes may be of a
> > complicated
> > nature. When the frequency is not too high (i.e.
> > when it is below
> > the cut-off frequency of the first higher-order
> > mode) only the
> > (quasi-)TEM mode will be propagating, while all
> > other modes are
> > attenuated (likely with an exponential decay).
> > Typically, these
> > decaying modes tend over a longitudinal region of
> > the order of
> > one (or a few) 'typical' dimension of the transverse
> > direction
> > (e.g. the width of the microstrip line). Looking at
> > this phenomenon
> > from a circuit point of view, the scattered
> > (quasi-)TEM mode appears
> > as a wave, reflected by the discontinuity, while the
> > higher-order
> > modes give rise to lumped 'excess' impedances. In
> > fact, these
> > 'excess' impedances represent all of the energy in
> > the higher-order
> > modes. When no losses (or radiation) occurs this
> > energy is purely
> > reactive and the corresponding 'excess' impedances
> > are reactive
> > as well (only imaginary part, such as capacitances,
> > inductances, ...).
> > As the frequency plays a role in the attenuation of
> > the higher-modes
> > (as well as in the (quasi-)TEM mode which is
> > dispersive as well)
> > these circuit elements will exhibit a frequency
> > dependence also.
> >
> > Good luck and best regards,
> >
> > Dr. Ronald De Smedt
> > Alcatel Bell, F. Wellesplein 1, B-2018 Antwerpen
> > (Belgium)
> >
> >
> > Charles Harrington wrote:
> > >
> > > Hello,
> > >
> > > When a TEM (or quasi-TEM) signal propagating on a
> > > microstrip trace meets a discontinuity (e.g., a
> > via)
> > > on a substrate at microwave frequencies,
> > higher-order
> > > modes are excited at the discontinuity, leading to
> > > reflections.
> > >
> > > Can anyone tell me exactly what kind of modes are
> > > excited and why they are excited. How far away
> > from
> > > the discontinuity can they propagate and how does
> > this
> > > depend on the operating frequency.
> > >
> > > I'm a postgraduate student currently working on
> > > discontinuities on PCB. My search for relevant
> > > literature led me to this list. So, I'll
> > appreciate
> > > any help.
> > >
> > > Charles.
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