RE: [SI-LIST] : skin effect

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From: Zabinski, Patrick J. ([email protected])
Date: Thu Aug 31 2000 - 12:08:54 PDT

Similarly, there are two subtle differences to mention as well:

* given the same perimeter, the DC resistance of a 'square'
trace will be lower than that of a flat/thin trace. If you're
frequency of operation is low, it is advantageous to use
thick/narrow traces.

* given the same perimeter, the AC resistance of a square
trace varies more with frequency than that of a flat/thin
trace. This one probably needs a bit more explanation ...

As you are aware and can see in the previous postings, the
skin effect essentially pushes the currents to the edges
of the conductor. As a specific frequency, if the skin
depth (per Ray's posting) is deeper than the thickness of
the trace, then you can approximate the current density
across the conductor as constant. Looking at it
a different way, if the trace thickness is thinner than
a nomimal skin depth given by the common formulas, the
effective resistance will be the same as the DC resistance.

So, at "very-high" frequencies, given the same perimeter, a narrow/thick
trace will have approximately the same loss as a
wide/thin trace. However, the narrow/thick trace
starts out at DC with a lower resistance/loss, so it
tends to vary more over freq than the wide/thin trace.

For most systems, I think this discussion is likely to be
in the noise, but for systems with very long lines where
dispersion is a big factor, it is often advantageous to
use wide/thin traces to keep the resistance (and inductance
and prop delay) constant over freq.


> Another factor not directly mentioned yet, is that increasing
> the trace
> width also widens the HF return path, decreasing its HF resistance ...
> whereas increasing the trace thickness doesn't.
> Andy

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