Sergio,

Try www.ideaconsulting.com. They have some on-line impedance calculation
pages.

IPC-2141 has some simple formulas and the specification for TDR (time domain
reflectrometry) impedance measusrments. It also specifies the standard
impedance test coupon for the PCB.

Johnson & Graham "High-Speed Digital Design" has chapter on transmission
line and formulas in appendix C. I've cast these into C programs which I can
give you. (contact me off line.)

National Semiconductor's  app note AP-905 shows some formulas for
differential impedance which are reportedly widely used but can have as much
as +/-10% error.

I am now using Hyperlynx which is an expensive ($20K+) package which
proports to use field equations (I assume Maxwell's) to calculate impedance
directly from the trace geometry and PCB bulk parameters.

Ray Klein

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