# Re: [SI-LIST] : Upper limit of interplane capacitance

From: Douglas C. Smith ([email protected])
Date: Fri Jun 09 2000 - 14:44:30 PDT

Hi All,

I am not a guru on this topic either, however I have thought
that there is more than a radial transmission line here, in
that the two dimensional transmission line has lots of funny
mid-plane loads in the form of bypass capacitors that give
reflectons (I am talking of power to ground plane here).
That combined with the open sides would make for a driving
point impedance that would should be quite lumpy with
frequency. I am ignoring the lossy loads of the devices
themselves which are another set of complicating factors.

Does anyone have experimental data they have taken on this
handy?

Doug (Smith)

Doug McKean wrote:
>
> "Chan, Michael" wrote:
> >
> > I would like to point out that what would be the impedance look like when
> > you looks at it from the center of the two plates viewing from the top? Can
> > you still qualify it as a rectangular wave guide as the wave is spread out
> > in
> > 360 degree other than in one particular direction. Instead of calling it the
> >
> > traditional "characteristic impedance" I would prefer to see it as " driving
> >
> > point impedance". Any comment from any guru ????
>
> The "equations" come out the same. Just as if you
> were asking if there would be any difference with
> the characteristic impedance of a one dimensional
> transmission line at the end or in the center.
> Reality would dictate something different with the
> geometries and cutouts in the planes.
>
> But there's a couple of different issues going on
> with parallel plates structures and quite different
> in many ways. One structure is that the parallel
> plates make a transmission line, the other is that
> they make a waveguide. The transmission line supports
> electric and magnetic fields in the dielectric as
> currents move in the plates. The waveguide simply
> *guides* fields between them. either along the axis
> of the plates or by reflecting them off the walls.
>
> In the case of the plates constituting a transmission
> line, circulating currents say in the power plane would
> terminate at the edges. Since there's no termination
> at the edge, this would cause theoretically a complete
> positive reflection.
>
> Like ripples on a pond, these would create nodes at
> various points about the planes depending upon many
> factors geometry being one. Which is in fact the case.
> Termination of such could not be accomplished with
> a single point connection such as a resistor.
>
> This phenomena cannot be explained when considering
> the planes as a *waveguide*.
>
> - Doug McKean
>
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