To SI experts, especially those used Maxwell's equations in their books: The concept of voltage has been deeply engrained in our minds. It's the foundation of the circuit theory and electrical engineering at large. Recently, when collecting materials for a presentation advocating the importance of signal integrity, I realized that the concept of voltage is based on the assumption of the electric field being conservative. The general differential form of Maxwell's equations (the Faraday's Law part in particular) contains the dB/dt term that ruins the validity of curl {E} = 0. There are two categories of arguments: (1) With the integral form of the Faraday's Law, we can treat the time-varying magnetic flux term as an electromotive-force (like battery), hence fix the KVL, the general idea of voltage is still being the line integral of the electric field between two points. (2) Switching to the frequency-domain, the surface integral of the B-field is related to the characteristic dimension of the system (D) and the operating frequency. It goes with some hand-waving arguments, and the claim is that the unfriendly term is nearly zero when D << wavelength. This argument also goes hand-in-hand with validity of lumped vs. distributed element modeling. I have also observed that in a multi-conductor transmission-line (MTL) system where the TEM mode is propagating, the E-filed is conservative on each cross-section, thus voltage is well defined between a signal conductor and the common-reference conductor. Here, the general electrodynamic Maxwell's equations are all satisfied. The MTL theory explains why RF/microwave testing has to be done using co-ax cables. It also makes sense why 'port' (instead of circuit node) voltage is used in constructing S-parameters. If the second explanation holds true, I am really worried about what the frequency limit is before we get into trouble with freely applying knowledge developed with DC or low-frequency circuits. Any way, looking for a better answer or proof on the validity of the concept of voltage. Thanks ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu