Characterization of unknown linear systems based on measured CW amplitude

A simple method known in network theory is used to determine the complete characteristics of an unknown linear system from a given CW (continuous wave) magnitude response only. These characteristics include possible different transfer functions, their phases, and the corresponding impulse responses. Only one transfer function has minimum phase. The main achievement is to deduce an approximate squared-magnitude function in the form of a ratio of two even polynomials based on the outstanding features, such as resonant frequencies and bandwidths, contained in the given measured magnitude. The remaining procedures for obtaining the complete system characteristics are exact. It is also shown that the minimum-phase case, through its associated impulse response and accumulated energy content, constitutes the most conservative estimate for the initial threat to the system by an external unfriendly source