[SI-LIST] : Attenuation and Delay on a PCB Trance

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From: Paul Levin (levinpa@earthlink.net)
Date: Thu Apr 12 2001 - 16:31:49 PDT


To all concerned,

Besides the attenuation effects on PCB traces, don't forget that the
propagation
delay is a function of frequency EVEN IF EPSILON-R IS CONSTANT.

Few people realize that skin effect resistance is only a portion of the
skin effect
impedance. The Rs figure must really be multiplied by (1+j). This
inductive
reactance means that there is a bit more inductance (diminishing as the
frequency
increases.) Because of this, lower frequencies propagate more slowly
than
higher frequencies.

This effect has nearly disappeared by the time one gets to 1 GHz, but 1
and 2
Gbps data signals have frequency components much lower than a GHz.
Consider
Fibre Channel's "alternating disparity K28.5 pattern." This 20 bit
pattern has
quite a bit of energy at 1/20th of the data rate. Some others have told
me that
they have been able to create some (admittedly pathological) data
patterns
that resulted in even lower frequency content.

Some years ago I computed the Fourier transform of the entire pattern
and
ran the harmonics through both the attenuation and phase characteristics
of
microstrip. It surprised people how well I could simulate the
asymmetrical
eye pattern. The analysis worked for shielded, twisted pair, too.

Paul Levin
Senior Principal Engineer
Logic Innovations, Inc.
A Xyratex Company

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