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

Douglas McKean (*dmckean@corp.auspex.com*)

*Mon, 30 Aug 1999 15:26:07 -0700*

At 03:35 PM 8/30/99 -0400, Ingraham, Andrew wrote:

*>>>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. *

*>>*

*>>I'm not convinced of that. In a truly lossless line, *

*>>characteristic impedence Z0 = [ L0/C0 ]^0.5 where L0 *

*>>and C0 are per length parameters of inductance and *

*>>capacitance respectively. So that theoretically, a *

*>>2 inch trace should have the same CI of a 20 inch trace. *

*>*

*>Consider inductance. Each infinitesimal segment of a line has an L per*

*>length, which includes both self and various mutual inductances to all its*

*>nearby segments (of the same t-line). Together, they make up the net Lo per*

*>unit length, for an infinite line.*

*>*

*>Now truncate the line. You no longer have the mutuals off the truncated*

*>end.*

Thanks for the come back Andy. It must truly be

a Monday. I'm stuck on lumped parameters. I'm

thinking that if the mutual inductance/length is

included, then it follows suit with the cap/length

or the induct/length.

Doug

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