[SI-LIST] Re: Discontinuities on PCB
- From: Ronald.De_Smedt@xxxxxxxxxx
- To: ch_harrington@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 12 May 2005 13:46:55 +0200
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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