[SI-LIST] Re: Propagation delay difference
- From: "Ihsan Erdin" <erdinih@xxxxxxxxx>
- To: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- Date: Wed, 14 Mar 2007 17:50:26 -0400
Ken,
First, I have to make a correction to my earlier statement concerning
the line width. A generic statement like "the finite size conductor
circumference" is more appropriate. I think you'd agree that the
fundamental assumption with the transmission line theory is the TEM
mode of propagation along the axis of the (ideal) lines, which
requires all the electric and magnetic field components to be confined
to the transverse plane. In that case, all propagation modes will have
the same velocity. In reality (PCB tracks, cable wires, etc.) however,
the line circumference is not infinitely thin as the theory assumes.
With this geometry you can't expect the current to be confined to the
axis of the line. Any non-axial current component will give rise to an
H-field which is not on the plane transverse to the direction of
propagation any more. Now, we talk about a quasi-TEM mode of
propagation which also applies to inhomogeneous media, lossy
conductors, etc. Does this make sense?
Regards,
Ihsan
On 3/14/07, Ken Cantrell <Ken.Cantrell@xxxxxxxxxxxxxxxx> wrote:
> Ihsan -
> Can you elaborate? I don't see how the width effects the mode velocities.
> Thanks,
> Ken
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ihsan Erdin
> Sent: Wednesday, March 14, 2007 9:41 AM
> To: steven.d.corey@xxxxxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Propagation delay difference
>
>
> Steven,
>
> A stripline structure doesn't support a pure TEM mode of propagation
> even in a lossless homogeneous medium -owing to its width- hence the
> difference in the mode velocities.
>
> Regards,
>
> Ihsan
>
> On 3/13/07, steven.d.corey@xxxxxxxxxxxxxx <steven.d.corey@xxxxxxxxxxxxxx>
> wrote:
> > Hi Ihsan -- all TEM waves propagate with the same velocity in a system
> > of multiconductor transmission lines in a homogeneous medium, as
> > determined by the properties of the medium. This is true for any number
> > of coupled lines -- two coupled striplines is just a special case.
> >
> > This should make physical sense -- the velocity of a wave traveling in a
> > homogeneous, linear, isotropic medium can not be changed by reorienting
> > its fields. Since a TEM wave travels down a transmission line through
> > such a medium with no obstructions, we should expect its velocity to be
> > that of the dielectric medium. Reorienting its fields within the
> > transverse plane, as we do by changing from even to odd excitation, or
> > any combination of the two, will not change the velocity of propagation.
> > You can find all the relevant mathematics in "Analysis of Multiconductor
> > Transmission Lines" by Clayton R. Paul.
> >
> > Of course the ideal TEM case is never exactly realized in practice,
> > since there are always some non-TEM effects -- conductor losses, finite
> > lengths of the lines, lines turning corners, finite extent of the
> > dielectric, the list goes on and on. But in pure theory, all of the
> > modal velocities (if we even choose to call them separate modes) are
> > identically equal.
> >
> > -- Steve
> >
> > ------------------------------------------------------------------------
> > -
> > Steven D. Corey, Ph.D.
> > Principal Engineer
> > Tektronix - Enabling Innovation
> > =20
> > http://www.tdasystems.com
> > http://www.tektronix.com
> > =20
> > email: steven.corey@xxxxxxxxxxxxx
> > phone: (503) 627-6816
> > fax: (503) 627-2260
> > ------------------------------------------------------------------------
> > -
> > =20
> >
> > >-----Original Message-----
> > >From: si-list-bounce@xxxxxxxxxxxxx=20
> > >[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Ihsan Erdin
> > >Sent: Friday, March 09, 2007 3:50 AM
> > >To: jbtera77@xxxxxxxxx
> > >Cc: si-list@xxxxxxxxxxxxx
> > >Subject: [SI-LIST] Re: Propagation delay difference
> > >
> > >Jongbae,
> > >
> > >Even in an homogeneous coupled stripline structure, the wave speed is
> > >not the same for even and odd propagation modes, hence the difference
> > >between common and diff. mode prop. delays. For cases where mutual
> > >coupling is very small they get close but theoretically there'll be a
> > >difference. The same reasoning applies to the coupled microstrip
> > >lines.
> > >
> > >Regards
> > >
> > >Ihsan
> > >
> > >On 3/9/07, jbtera77@xxxxxxxxx <jbtera77@xxxxxxxxx> wrote:
> > >> Hi, all
> > >> As all know, in homogeneous strip line, the propagation=20
> > >speed is the same
> > >> for both differential and common mode.
> > >>
> > >> However, in inhomogeneous microstrip line, the propagation speed of
> > >> differential mode is faster than that of common mode.
> > >> In specific, the value of Cm/C is higher than Lm/L in inhomogeneous
> > >> microstrip line while the values are same in homogeneous strip line?
> > >>
> > >> Could somebody let me know why it physically is?
> > >>
> > >>
> > >> Thanks in advance,
> > >> Jongbae Park.
> > >>
> > >>
> > >>
> > >> --
> > >> --------------------------------------------------------Ph.D=20
> > >Candidate
> > >> StudentTerahertz Interconnection & Package Laboratory,Department of
> > >> Electrical Engineering, Korea Advanced Institute of Science=20
> > >and Technology
> > >> (KAIST),373-1, Kusong-dong, Yusong-ku, Taejon 305-701,
> > >>=20
> > >KoreaFax)+82-42-869-8058Tel)+82-42-869-5458Mobile)+82-11-9787-7
> > >966E-mail)
> > >> pjb77@xxxxxxxxxxxxxxxxxxxxxx URL)
> > >>=20
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