On the Characterization of a Bandpass Nonlinearity by Two-Tone Measurements

The characteristics of a bandpass nonlinearity are expressed in this paper by a finite series expansion of the complex envelope transfer function. This function is commonly acquired by direct measurement of the single-carrier envelope characteristics. This paper describes an alternative method whereby the complex envelope transfer function is obtained by measuring the power of the intermodulation products generated by two-tone signal inputs. This approach is known to permit a more accurate characterization than the other method, particularly in the cases of mild nonlinearities. The fundamental problem that has to be overcome in order to establish the complex envelope characteristic from power measurements alone, is the determination of the phase of the measured intermodulation components. The solution, as developed in this paper, is to measure the two-tone spectrum at two power levels when the nonlinearity only exhibits amplitude distortion, and at three power levels when the nonlinearity exhibits AM-to-PM conversion in addition to amplitude distortion.