The present invention is concerned with a method of determining stationary signals for the diagnostics of an electromechanical systems in which electrical rotating machinery is used and in which at least one electrical or mechanical signal is measured during an operation of the electromechanical system. The method is used especially for condition monitoring of electric motors and generators. The method consists of measuring an analog waveform signal (S) of the electromechanical system and then manipulating that signal in various ways to obtain a frequencies spectrum, from which a vector of interest frequencies and corresponding vector of amplitudes are extracted to diagnose the electromechanical system.