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

Ingraham, Andrew ([email protected])
Mon, 30 Aug 1999 15:35:00 -0400

>>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 /
>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

It is no longer a TEM wave near the ends of a line. All assumptions about
uniform Co and Lo are theoretically invalid. To an engineer's
approximation, it may still behave about the same; but not if you look
closely enough.

Andy Ingraham

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