Model Fitting Using Both TDR and Frequency Responses

Fitting a circuit model to measured data is usually accomplished by matching the model\´s calculated \´S\´ parameters to the target\´s \´S\´ parameters as measured on a VANA over the model\´s operating frequency range. For electrically " long" structures, several wavelengths long at the upper modelling frequency and poor starting values, the conventional technique leads to convergence difficulties. Using the IFT to effectively obtain the time domain step reflection response and matching this to the target\´s similarly transformed response, leads to better starting values and surer convergence. Examples are given, run on a low cost, commercially available circuit analysis program, of optimization using frequency domain only and time/frequency domain switching, for several simple networks. Some of the resolution limitations inherent in the method and choice of mode switching points are discussed.