The Effect of Quadrature Modulator Nonlinearity on a Digital Baseband Predistortion System

Digital baseband predistortion system uses a quadrature modulator (QM) to transfer the predistorted baseband signal to RF. Previously the effect of linear QM errors namely, DC offset, phase imbalance and amplitude imbalance have been analyzed. However the baseband amplifiers and the QM introduce also nonlinear distortion, that is partly dependent on the QM error correction. This paper analyses the effect of this distortion on the PA output with and without adaptation feedback. The results show that if third order distortion dominates at the baseband, adaptive predistortion function can reduce the distortion. The results also show that matching of the third order baseband nonlinearities between different branches is important to minimize the nonlinearity at the PA output but the matching of the second order is not so important