[SI-LIST] Re: Discontinuity Location and S-Parameters
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: Avtaar Singh <avtaarenator@xxxxxxxxx>
- Date: Tue, 21 Dec 2010 11:00:42 -0800
Avtaar,
if both your driver and receiver provide matched termination, then the
placement of a single discontinuity/parasitic on the line has no impact on
S21 - it will always be the same. (This is not true if two or more
parasitics are present - their _relative_ location with respect to each
other will affect the shape of S21 over frequency. Same goes for the case
where driver and/or receiver to not provide matched termination).
As for the length influence, bandwidth due to dielectric losses will
decrease linearly with line length, but quadratically for skin losses (you
will always have a mixture of both loss types unless your dielectric is
air). But most likely the length variation is small against the total line
length so that is a minor contributor to the variation.
For multiple discontinuities the worst case assumption would be that all
losses (S21) of all discontinuities simply add up. That does not help much
for pre-emphasis though. Also, today's pre-emphasis mostly helps with
"real" (dissipative) line losses (skin effect and dielectric losses) which
have a relatively easy to predict behavior over frequency (typically
monotonically increasing with frequency, at least over the frequency range
spanned by today's digital signals), but not for losses due to reflections.
Wolfgang
Avtaar Singh
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[SI-LIST] Discontinuity Location
12/20/2010 10:28 and S-Parameters
PM
Gurus,
I am trying to simulate a multiple channel serdes link, spanning across a
switch cards, backplane, and line cards.
I am trying to create a thumb rule on how much will the discontinuity (read
connector) location affect the insertion loss (trying to decide on
pre-emphasis and equalization). Is there a back-of-the-envelope trick to
make an assumption on how much will the connector placement affect the
S-params as compared to etch length variations?
--
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Regards,
Avtaar Singh
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