Re: [SI-LIST] : PCB impedance variation v.s. layout pattern

S. Weir (weirsp@a.crl.com)
Sat, 28 Aug 1999 22:04:36 -0700

Doug,

I believe there is an error in your conclusion on the inductance. I
believe the "pure" view of a transmission line is an infinite, unvarying
geometry which is uniform to all wavelengths of interest. It is with that
assumption that if we cut the transmission line, we get infinity less a
finite, which ultimately leads to the differential view of C / X, and L /
X. When a meander is added, it alters the geometry. If the curvature is
tight enough, then the AC field distribution will be noticeably different
in the region of the meander than for some distant segment. We observe
this as an increase in the apparent capacitance per unit length in the
region of the meander. So, for this effect the impedance will go down. I
do not see a complementary effect which will drive the inductance up. If
anything, the fact that the magnetic field near the start of the meander
now couples more with trace further down and at an angle other than 0
degrees implies that the inductance goes down per unit length in this
region as well.

Regards,

Steve.

At 02:11 PM 8/28/1999 -0700, you wrote:
>
>Sorry, there's a classic case of thinking one
>thing and saying another. I was thinking of
>the "plate distance" adding. The "capacitance"
>of caps in series is 1/Ctot = 1/C1 + 1/C2 + ...
>
>What's interesting with the case originally asked
>is that characteristic impedence is theoretically
>immune to length. Thus, even if the meandering
>does add more capacitance from it's relationship
>with the ground plane, it also adds more inductance
>because of the increased length. Since characteristic
>impedence is dependent on the per length values of
>capacitance and inductance, it would require IMHO,
>the addition of another parameter in order to
>change the CI.
>
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