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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