Spectral sensitivity of power amplifiers to quadrature modulator misalignment

Misalignments in the quadrature modulator feeding a nonlinear high power amplifier can produce distortion products that lead to spectrum leakage into adjacent channels. Expressions are derived relating the out-of-band spectra to these misalignments and to the complex coefficients of the power series describing the amplifier. For most applications the adjacent channel leakage 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 modulations (CPM). A technique for estimating the spectrum for the above situations is described. It involves summing the weighted spectrum of each distortion product. The weighting coefficients can be varied to account for amplifier characteristics, backoff, and misalignment error. The analysis is applicable to linear or continuous phase modulations with or without predistortion. The spectrum estimate is particularly good for continuous phase modulated signals where the error is normally much less than 1 dB