Minimizing Spectral Leakage of Nonideal LINC Transmitters by Analysis of Component Impairments

LInear amplification with Nonlinear Components (LINC) is a distortion-free transmitter architecture under ideal conditions. However, in a realistic transmitter, imbalances such as dc offset, differential gain, and differential phase shift present in the quadrature modulators result in generation of out-of-band spectral components, even assuming that both amplifiers have identical characteristics. The level of distortion not only increases with the magnitude of the imbalances but also depends, to a large extent, on their relative phase arguments. The contribution of this paper is to provide a simplified characterization of the spectral leakage in the LINC transmitter in terms of quadrature modulator imbalances, with identical or mismatched amplifiers. This characterization allows identification of the transmitter with minimum spectral leakage under the constraint that the quadrature modulators perform equally well when used individually, i.e., in a transmitter composed of just the quadrature modulator followed by an amplifier. With reasonably small imbalances in the quadrature modulators and realistic amplifier mismatch, the spectral leakage at the output of a LINC transmitter driven by typical modulation schemes can be reduced by up to 10 dB, simply by a wise pairing of modulators