[SI-LIST] Re: Help Explaining Microstrip
- From: "Mark Randol" <Mark.Randol@xxxxxxxxxx>
- To: "SI-LIST Reflector" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 19 Oct 2007 08:26:40 -0700
It may also be worth pointing out that what contains the EM field in the
transmission line (microstrip, coax, waveguide, et al) is the surface
currents that flow in the 'outer wall' of the transmission media due to
those field. The point could be argued that the field is created by the
currents since one can't exist without the other. =20
~"You pays your money and chooses your frame of reference." - unknown
That is unless we're talking about something like a light pipe
(dielectric waveguide). That contains the EM energy by bending or
reflecting it back into the middle by varying dielectric properties.
Probably my terminology isn't exact here since most of what I know about
them is that they exist and I've seen them. =20
--
Mark Randol, RF Evaluation & Application Engineer
ON Semiconductor
901 S. Mopac Expressway
Barton Oaks 4, Suite 343
Austin, TX 78746
512-329-5640 (voice), 512-329-8151 (FAX)
=20
=20
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx=20
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of istvan novak
> Sent: Friday, October 19, 2007 7:23 AM
> To: erdinih@xxxxxxxxx
> Cc: levinpa@xxxxxxxxxxxxx; SI-LIST Reflector
> Subject: [SI-LIST] Re: Help Explaining Microstrip
>=20
> Ihsan,
>=20
> I am not sure I would call the return current fictitious, at=20
> least not when we do speak about signal current at the same=20
> time. What do we do then when we have the hypothetical case=20
> of a floating symmetrical differential wire pair, where=20
> 'signal wire' and 'return wire' look and behave the same?
>=20
> Respectfully,
>=20
> Istvan Novak
> SUN Microsystems
>=20
> Ihsan Erdin wrote:
>=20
> >Paul,
> >
> >Unlike the trace current which is forced by the source,=20
> return current=20
> >is a fictitious entity which mainly serves for pedagogical purposes.
> >In this regard, it is not correct to treat the signal-reference=20
> >conductor system like a differential structure in which the=20
> current is=20
> >forced with opposite polarities in both lines. A microstrip -or in a=20
> >broader sense- any transmission line is a waveguide that=20
> entraps the EM=20
> >energy between the reference and the signal conductor and directs it=20
> >along the line axis. What we call "line characteristic impedance"
> >is indeed nothing but the "wave impedance" in EM theory. The current=20
> >flows on the signal conductor only, thus supporting the transverse=20
> >electric and magnetic field components. All SI (and EMI) issues like=20
> >crosstalk, discontinuity effects, etc. can be rigorously=20
> explained with=20
> >the medium and field interaction. This, however, requires a=20
> >considerable grasp of EM and microwave theory. The "return current"
> >phenomenon is a short cut way of explaining SI issues to=20
> people who may=20
> >have little exposure to EM theory.
> >
> >Regards
> >
> >Ihsan
> >
> >On 10/17/07, Paul Levin <levinpa@xxxxxxxxxxxxx> wrote:
> > =20
> >
> >>Dear SI-LIST'ers,
> >>
> >>I'm working on a presentation to explain transmission line to=20
> >>non-engineers and I find myself stumbling over some of the basics.=20
> >>(There's nothing like explaining something to bring out all of the=20
> >>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=20
> >>you have two conductors carrying current in the same=20
> direction, they=20
> >>would would to pull in close to each other whereas if you had two=20
> >>conductors carrying current in opposite directions, they=20
> would want to=20
> >>separate.
> >>
> >>If one were to apply just these observations to microstrip,=20
> you would=20
> >>expect to see all of the trace current bunched on the side=20
> away from=20
> >>the ground plane and the return plane current in two=20
> bunches to either=20
> >>side of the trace and as far away from the trace as=20
> possible, if not=20
> >>on the bottom.
> >>
> >>Of course, this is almost exactly opposite from what we=20
> know happens.
> >>
> >>What is the force that overcomes Oersted and Ampere and causes the=20
> >>trace and return currents to be so heavily attracted to each other?
> >>
> >>Thank you in advance.
> >>
> >>Regards,
> >>
> >>Paul Levin
> >>Senior Principal Engineer
> >>Xyratex
> >>
> >>
> >> =20
> >>
>=20
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