Alright, I'll take a stab ...
At 02:30 PM 5/12/99 -0700, Allen Tudor wrote:
>
>Greetings group,
>
>I am trying to draw a parallel between transmission line theory and
radiated emissions.
>
>From what I understand, a transmission line can be terminated at the
source or the load with an impedance that equals the characteristic
impedance of the transmission line itself.
The important thing to remember is that once the
signal energy has reached the far end of the trace,
you want to dump that energy into a resistive load.
Loading a line at the source end ... I'm not familiar
with that. Even so, once the signal gets to the unterminated
far end of the trace, there's an impedance mismatch and you
will get reflection. Ideally, you will get 100% reflection.
Tau(reflected) = E(reflected)/E(incidence)
= (RLoad - RSource)/(RLoad + RSource)
RLoad = infinity (unterminated) means 100% reflection.
Tau(transmitted) = E(transmitted)/E(incidence)
= (RLoad^2)/(RLoad + RSource)
>With this in mind, consider this scenario. A printed circuit card drives
a clock signal down a trace on a backplane. The length of the backplane
trace is long enough to be considered a transmission line. The driver on
the printed circuit card is located within ½ inch of the edge connector
(mating with the backplane) and is terminated with an impedance equal to
the characteristic impedance of the backplane trace. However, the
backplane trace is open ended (there is nothing connected to the end of the
trace). Transmission line theory says the signal integrity will be
maintained in this case.
Forgive me on this, but I don't see how.
I'm sort of a purist. You'll get 100%
reflection under ideal conditions.
>Now for the questions:
>
>(1) How much, if any, of the energy will be radiated into free space when
it gets to the end of the open transmission line? To me, this looks like a
monopole antenna. I don't have a very good understanding of antenna
theory, so this could very well be an invalid assumption.
Of the cuff? Unknown. There's too many variables.
Single wire antenna (monopole) embedded in FR-4 ...
>(2) If radiation does take place as stated above in question (1), which is
better for reducing the radiation, termination at the source or termination
at the load of the transmission line, or does it matter?
Termination at load end.
>(3) If the characteristic impedance of the trace on the printed circuit
card differs from the characteristic impedance of the trace on the
backplane, how is this handled? Is a termination needed at each end in
this case?
I've seen small resistors put
in series with the trace.
Keep in mind that this "characteristic impedance"
is not really a purely resistive thing.
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