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 --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------