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

Ingraham, Andrew (Andrew.Ingraham@compaq.com)
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 /
>>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.

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.

Regards,
Andy Ingraham

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