[SI-LIST] Re: skin effect
- From: "Gil Gafni" <gil.gafni@xxxxxxxx>
- To: <setavala@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 26 Jul 2004 08:18:10 -0700
Dear Santosh
Mr. Tony Dunbar described the phenomenon very accurately; with your
permission I will try to shed some light on the practical side of it,
from a system design point of view.
Skin effect "hits" you in two arenas -
1. Traces -
A) Here as Tony explained, the higher the frequency the less "skin" we
get (see bellow). Less 'skin' means that the same amount of electrical
charges will have less of cross section area to go through, which means
a weaker currents (current are directly related to the cross area).
Weaker current means that the resistance of the line has increased...
Cross section of a trace
---------------- --------------------
| | | -------------- |
| -------- | | | | |
| | | | | | | |
| -------- | | | | |
| | | -------------- |
---------------- --------------------
Low frequency High frequency
B) Since there is an effect on the current, and current is also a major
contribution to inductance (through current loops...), it changes the
inductance of the line. Here the relationship is reverse, therefore with
higher frequencies, we will get lower di/dt hence higher L.
2. Power planes
Again the lower the frequency - the deepest the skin. So, in some
situations, if your power plane is not thick enough, and your
frequencies of operation are low enough, energy of a return path will
affect the other side of the plane, causing cross talk...
Note -
Since it is a frequency related phenomenon, it is also important to
understand the relationship of the rise time and this effect. Let's
assume an interface working at 100MHz with hypothetical rise time of
1nS. Your highest frequency of interest is not 100MHz... but rather
1ns/0.35 which is about 2.85GHz... (This is the F knee) so now you have
more frequencies to think about...
Gil Gafni
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Santhosh E P (setavala)
Sent: Monday, July 26, 2004 5:31 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] skin effect
hi all,
can anyone give a definition for skin effect in pcb... how does this
change
according to the frequency of the signals?
is skin effect comes into picture on for the micreostrip not for
stripline,
if then why?
regards
santhosh
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