1. A computing device, comprising: a storage device; at least one processor; and a passivity enforcing unit comprising one or more computerized codes, which are stored in the storage device and executable by the at least one processor, the one or more computerized codes comprising: a parameter reading module operable to read a scattering parameter (S-parameter) file from the storage device, wherein the S-parameter file lists S-parameters measured at ports of a circuit at different signal frequencies; a vector fitting module operable to create a non-common-pole rational function of the S-parameters by applying a vector fitting algorithm to the S-parameters; a passivity enforcing module operable to enforce passivity of the non-common-pole rational function if the non-common-pole rational function does not satisfy a determined passivity requirement; and an equivalent circuit generation module operable to generate an equivalent circuit model of the circuit according to the non-common-pole rational function which satisfies the determined passivity requirement by enforcing passivity.