1. A method of determining operating characteristics of a signal generator by a measuring device, the method comprising: performing a first set of measurements of an output signal generated by the signal generator and a corresponding reflected signal at an output port of the signal generator, wherein the first set of measurements is performed over a plurality of frequencies and a plurality of amplitudes of the output signal; applying an external signal to the output port of the signal generator; performing a second set of measurements of the output signal and the corresponding reflected signal at the output port of the signal generator while the external signal is being applied to the output port, wherein the second set of measurements is performed over the plurality of frequencies and the plurality of amplitudes of the output signal, the external signal having the same frequency as the output signal for each measurement of the second set of measurements; and determining a set of coefficients describing the operating characteristics of the signal generator by processing results of the first and second sets of measurements through a non-linear model.