[SI-LIST] Re: Help Explaining Microstrip
- From: "Ihsan Erdin" <erdinih@xxxxxxxxx>
- To: levinpa@xxxxxxxxxxxxx
- Date: Fri, 19 Oct 2007 06:27:38 -0400
Paul,
Unlike the trace current which is forced by the source, return current
is a fictitious entity which mainly serves for pedagogical purposes.
In this regard, it is not correct to treat the signal-reference
conductor system like a differential structure in which the current is
forced with opposite polarities in both lines. A microstrip -or in a
broader sense- any transmission line is a waveguide that entraps the
EM energy between the reference and the signal conductor and directs
it along the line axis. What we call "line characteristic impedance"
is indeed nothing but the "wave impedance" in EM theory. The current
flows on the signal conductor only, thus supporting the transverse
electric and magnetic field components. All SI (and EMI) issues like
crosstalk, discontinuity effects, etc. can be rigorously explained
with the medium and field interaction. This, however, requires a
considerable grasp of EM and microwave theory. The "return current"
phenomenon is a short cut way of explaining SI issues to people who
may have little exposure to EM theory.
Regards
Ihsan
On 10/17/07, Paul Levin <levinpa@xxxxxxxxxxxxx> wrote:
> Dear SI-LIST'ers,
>
> I'm working on a presentation to explain transmission line to
> non-engineers and I
> find myself stumbling over some of the basics. (There's nothing like
> explaining
> something to bring out all of the glitches in what you were sure you
> understood!)
> I'm hoping that one of you may be able to supply the missing link.
>
> Nearly two hundred years ago Oersted and Ampere figured out that if you
> have
> two conductors carrying current in the same direction, they would would
> to pull
> in close to each other whereas if you had two conductors carrying
> current in
> opposite directions, they would want to separate.
>
> If one were to apply just these observations to microstrip, you would
> expect to
> see all of the trace current bunched on the side away from the ground
> plane and
> the return plane current in two bunches to either side of the trace and
> as far away
> from the trace as possible, if not on the bottom.
>
> Of course, this is almost exactly opposite from what we know happens.
>
> What is the force that overcomes Oersted and Ampere and causes the trace and
> return currents to be so heavily attracted to each other?
>
> Thank you in advance.
>
> Regards,
>
> Paul Levin
> Senior Principal Engineer
> Xyratex
>
>
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