== Calculating Scattering Parameters Using Prony's Method ==
[[EM.Picasso]] provides a special source type called '''[[Glossary of EMGlossary_of_EM.Cube's Excitation Sourcess_Materials,_Sources,_Devices_&_Other_Physical_Object_Types#Scattering Wave Port Scattering_Wave_Port | Scattering Wave Port]]''' that is specifically intended for computing the S-parameters of planar structures. This is done by analyzing the current distribution patterns on the port transmission lines. The discontinuity at the end of a port line (junction region) gives rise to a standing wave pattern in the line's current distribution. From the location of the current minima and maxima and their relative levels, one can determine the reflection coefficient at the discontinuity, <i>i.e.</i> the S<sub>11</sub> parameter. A more rigorous technique is Prony’s method, which is used for exponential approximation of functions. A complex function f(x) can be expanded as a sum of complex exponentials in the following form:
:<math> f(x) \approx \sum_{n=1}^N c_i e^{-j\gamma_i x} </math>