The effect of non-linear amplifiers and quadrature modulators on the out-of-band spectrum of digital modulations

Expressions are derived relating the out-of-band spectra to the misalignments in a quadrature modulator and to the complex coefficients of the power series describing the amplifier. For most applications the adjacent channel interference (ACI) is proportional to the square of the misalignment error (carrier leak, differential gain,differential phase and level error) except in the case of some higher order products of continuous phase modulation. A technique for estimating the spectrum for the above situations is described. It involves summing the weighted spectrum of each distortion product. The analysis is applicable to linear or continuous phase modulation (CPM) with or without predistortion. The spectrum estimate is particularly good for CPM signals where the error is normally much less then 1dB