RE: [SI-LIST] : skin effect

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From: Michael Khusid (mkhusid@sitaranetworks.com)
Date: Thu Aug 31 2000 - 11:50:55 PDT


Hello Ray and Muhammad,

I am sure I am not the only person who will notice that, so I won't spam
Si-list.

0.7 mils is 0.018 mm, not 0.18mm, that is, the skin effect depth of 0.002mm
is noticable.

Also, I am not 100% certain what formula Ray used for the skin effect depth.
If that is the formula for infinitely thick metal and a wave normally
insident on this metal (which a standard definition of skin-effect depth),
it will not give a good prediction for a realistic current distribution in
the stripline case. There are two reasons: first, in the stripline the
propagation of field is parallel to the metal, not perpendicular, and
second, the thickness comparable to penetration depth changes the current
distribution, and thus, the skin effect. I would love to give a book
reference where this issue is discussed in details

Lastly, for digital signals which have a very broadband spectrum, a skin
effect effect cannot be cleanly defined, for example, skin effect is heavily
frequency dependant. My favorite way to solve a skin effect problem is an
FDTD (finite difference time domain) solver which can actually uses Maxwell
equations directly and avoid approximations done to derive the
aforementioned skin effect formula. Practically any FDTD solver can solve
for current distribution inside a real conductor and calculate the resulting
skin effect loss.

(P.S. This is a view from purely high speed digital design, as indicated
above, microwave people will look at this problem from different
prospective)

Mike Khusid
Senior Hardware Engineer
Sitara Networks, Inc.
http://www.sitaranetworks.com

> -----Original Message-----
> From: Ray Anderson [mailto:Raymond.Anderson@Eng.Sun.COM]
> Sent: Thursday, August 31, 2000 1:32 PM
> To: si-list@silab.eng.sun.com
> Cc: msagarwa@san-jose.tt.slb.com
> Subject: Re: [SI-LIST] : skin effect
>
>
> Muhammad-
>
> I'll keep this very intuitive and non-mathematical
> because I don't
> feel like typing equations this morning.
>
>
> Consider that skin effect causes an effective increase in
> high-frequency
> resistance because the high-frequency current tends to flow
> mainly on the
> surface (or skin) of the conductor.
>
>
> At high frequencies the skin depth is very thin. For
> example at 1GHZ
> it is about .002 mm deep. Half ounce copper is about .7 mils
> thick (.18 mm).
> So the skin effect removes .002 mm of effective thickness
> from each side of
> the conductor .18mm - ( 2 x .002mm) = .176 mm . So doubling the copper
> thickness to 1 ounce material (1.4 mil or .36 mm) won't do
> you any good as a
> means to reduce the skin loss as the current is still flowing
> predominantly
> in the outer .002 mm of the conductor no mater how thick the metal is.
>
>
> However, by making the conductor wider you can reduce
> the skin effect
> because you increase the surface area of the conductor which
> provides more
> parallel "squares" of material in the conductor which reduces
> the effective
> resistance.
>
>
> -Ray
>
>
>
>
> > Hello Si Gurus,
> >
> > I just had an interesting discussion with my boss on "skin
> effect on pcb traces".
> > I was of the opinion that increasing the trace thickness
> from 1/2 oz. to 1oz. would help reduce the
> > skin effect but according to him skin effect does not
> reduce significantly with the increase in
> trace thickness.
> > He was of the opinion ( and also had some data to back him
> up) that skin effect is more dependent
> on the
> > width of the trace.
> >
> > I always thought that if one increases the overall
> perimeter of the pcb trace - regardless of
> whether it is done by
> > increasing the width or increasing the thickness - the skin
> effect would reduce. I would
> appreciate if somebody
> > could come up with a better explanation...
> >
> > Muhammad
> >
> > p.s.: when we talked about pcb traces we were talking about
> striplines in particular...
> >
> >
> >
> >
> >
> > Muhammad S. Sagarwala
> > Design Engineer
> > Schlumberger SABER
> > Ph. (408) 586 7065
> > Fax (408) 586 4668
> >
>
>
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