Vector Fitting
Constraints
Stable poles
In the case of transfer functions and transfer matrices, the code given in vfit.zip and MFTNNLS.zip are directly applicable to system identification as the obtained model satisfies the conjugacy requirement and the stable poles requirement. MFT-NNLS is intended for fitting square matrices as function of frequency.
Passivity
In the case of Y-, Z- or S-parameter modeling, one should in addition require that the model satisfies the passivity criterion. This is because interaction between the model and the connected network may otherwise cause an unstable time domain simulation. MFT-NNLS enforces passivity of the model by an iterative procedure whereby the residue matrices are modified while minimizing the change to the original model.