[SI-LIST] : Tangent on Long bus or star?
Hans Mellberg ([email protected])
Tue, 6 Apr 1999 11:31:08 -0700 (PDT)
The easiest way to "visualize" transmission lines and reflections, is
to think in terms of charges (q). When a signal (a lump of charges)
travels towards a termination (load) there is an equal and opposite
charge travelling in the same direction in the image structure. (Which
may be a plane, planes, adjacent traces(crosstalk), etc.) The equal and
opposite travelling charge is required to exist whether you intent to
have it or not. Laws of physics. When those charges reach the
destination, they will;
1) "Wrap around" right through the termination load (if Zo = 0) which
means that the charges will now have travelled through the termination
point and are now returning on the image structure with its image
(opposite polarity) returning on the signal line, (This mechanism
explains why the reflected voltage is of opposite polarity) or,
2)"Bounce back" (if Zo = infinity)which means that there was no
continuing path for the charges so they are "reflected" back the way
they came (same goes for the image charges, (This mechanism explains
why the reflected voltage is of the same polarity and why it reaches
twice the value(because during the reflection, the density of charges
at the measurement point is doubled) or,
3) a combination of both with partial "consumption" (if Zo is in
between) In the special case where Zo = Zt then all of the charges are
"consumed" (meaning converted to heat) Since it is not possible to have
a 0 impedance path there will be some "consumption" during the signal
path travel. At higher frequencies, dielectric effects begin to
"consume" additional charges (loss tangent (molecular
excitation/alignment) etc.))
Hope this helps
Hans Mellberg
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