[SI-LIST] Re: skin effect
- From: "Gil Gafni" <gil.gafni@xxxxxxxx>
- To: <andrew.c.byers@xxxxxxxxxxxxxx>, <gil.gafni@xxxxxxxx>, <setavala@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 26 Jul 2004 11:46:28 -0700
Andy, Tony et al
You are correct, and I was wrong about the increase/decrease of L (total
of internal and external). My bad ;-)
Thank you (and Tony) for pointing and for correcting that :-)
Gil
-----Original Message-----
From: andrew.c.byers@xxxxxxxxxxxxxx
[mailto:andrew.c.byers@xxxxxxxxxxxxxx]
Sent: Monday, July 26, 2004 9:39 AM
To: gil.gafni@xxxxxxxx; setavala@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: skin effect
Gil,
Maybe I misunderstood your post, but I feel the need to address your
following point:
<snip>
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. <snip>
Although you are correct about the effect on the current and the fact
that this affects inductance, it is not correct to say that inductance
increases with frequency. In fact, it does just the opposite. As current
crowds to the surface, you lose your internal inductance term and the
overall inductance is lower. Eventually you are basically left with only
the term referred to as external inductance, which is the value you
would get if there were no current penetration at all into the
conductors. Since they are related by the same skin effect action, the
resistance will begin to increase at the same frequency that the
indutance begins to decrease.
Check out Howard Johnson's latest book "High Speed Signal Propagation"
on page 66 (section 2.7) to get a clear explanation of this phenomenon.
regards,
Andy Byers
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Gil Gafni
Sent: Monday, July 26, 2004 8:18 AM
To: setavala@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: skin effect
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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