[SI-LIST] : Re: Skin Effect and Signal Dispersion

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From: Paul Levin (levinpa@ix.netcom.com)
Date: Tue Sep 05 2000 - 12:19:39 PDT


Dear All,

I wanted to suggest another reference that might aid in your
understanding
of these two related phenomena - Ramo, Whinnery and VanDuzer, "Fields
and Waves in Electronic Communications."

The book explains that the skin effect resistance is mirrored by an
equal
amount of inductive reactance. Since the resistance is increasing as
sqrt(f),
at least once the skin depth becomes a small portion of conductor depth,

this means that the equivalent inductance is dropping as a function of
freq.
Once this inductance is 'small enough,' the characteristic impedance
stabilizes at the nominal value; at low frequencies, this inductance,
which
adds to the line's geometric inductance, increases the characteristic
impedance and lowers the signal velocity.

Once upon a time (about two years ago,) I simulated some traces carrying

Fibre Channel's 'alternating disparity K28.5 pattern.' I did this by
doing a
fourier analysis of this 20-bit pattern and then applying the
attenuation and
phase delay predicted by Ramo, et al. to each harmonic. The resulting
eye pattern agreed with observed data, including a kink in the middle of

the transitions.

As I recall, the most significant Tpd changes for 1.4 mil trances and
28ga
conductors were in the first hundred MHz.

Happy reading and researching,

Paul Levin
Senior Principal Engineer
Logic Innovations

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