Hi Ray With reference to your posting earlier this year regarding n-ports etc., here is a thought on simulation of interconnects at high frequencies where the concept of common voltage reference nodes seems to become a bad recipe, thinking particularly of RF modelling of IC packages. As Khalil and Steer (paper cited below) have pointed out, the voltage between two points is undefined in general. This is an aspect of field theory which becomes relevant when the frequency is high enough that the phase delay between two points in a structure is a significant proportion of the wavelength. The significant proportion of course depends upon the application, so we can't define a specific threshold frequency even for a specific mechanical dimension. In analog or mixed-mode circuit designs, relatively small values of couplings or impedance may be significant, but such values might be ignored in a purely digital circuit. In addition, for a correct DC simulation of the operating point and power supply currents, together with broad band accuracy, the effect of internal inductance and frequency dependent resistance (both arising from skin effect and providing several percent effects) will need to be included. In an earlier posting, Ege Engin wrote this helpful comment: "If an S parameter matrix is implemented in a circuit simulator, it actually divides the rest of the circuit (all the other linear and non-linear elements) into groups, that are only coupled to each other by means of this S parameter matrix (due to the fact that an S parameter matrix represents a distributed circuit). Since the voltage drops between the local reference nodes in various groups are undefined, they can be connected to each other in an arbitrary manner." I would just add that in practice, from the s-parameters obtained by electromagnetic simulation or measurement, we have to formulate a polynomial or lumped-element solution to feed into the nodal transient circuit simulator (Spice or Spectre). Ege Engin's final sentence would then apply to the interconnection of the extracted n-port model with the chip schematic. In Khalil and Steer, "Circuit Theory for Spatially Distributed Microwave Circuits" (IEEE Transactions on Microwave Theory and Techniques, Vol. 46 No. 10, October 1998), we find: "The essence of the problem is that a global reference node cannot reasonably be defined for two spatially separated nodes when the electromagnetic field is transient or alternating. In this situation, the electric field is nonconservative and the voltage between any two points is dependent on the path of integration and, hence, voltage is undefined. This includes the situation of two separated points on an ideal conductor." So we see that each port requires its own separate local return pin in order to describe the distributed structure with sufficient accuracy over the required frequency range. Two or more ports can only use a common ground if they are physically close enough to one another (for the specific case). Finally, we make the arbitrary (?) decision to join the local ground(s) to the common ground and hope it's OK. From the network theory it seems OK, but is a little hard to swallow. Any comments? Best wishes, Geoff ______________________________________________ Geoff Stokes Applications Engineer, Signal Management Group Zetex plc Lansdowne Road, Chadderton, Oldham, OL9 9TY, UK Tel direct: +44-161-622-4857 Switchboard: +44-161-622-4444 Fax: +44-161-622-4469 http://www.zetex.com <http://www.zetex.com/> e-mail: gstokes@xxxxxxxxx _________________________________________________________ Zetex Semiconductors - Solutions for an analog world EID Award Winners for 'Best Use of Technology' 2003 for the AcoustarTM ZXCW8100 End-to-End Digital Audio Amplifier Controller http://www.zetex.com _________________________________________________________ ###################################################################### E-MAILS are susceptible to interference. You should not assume that the contents originated from the sender or the Zetex Group or that they have been accurately reproduced from their original form. 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