Spectral sensitivity of LINC transmitters to quadrature modulator misalignments

The effects of misalignments in quadrature modulators on the LINC transmitter performance are studied by means of analysis and simulation. Ideally, each branch in the LINC system carries a constant amplitude signal allowing nonlinear high efficiency amplifiers to be used without generating intermodulation distortion (IMD). Misalignments in the quadrature modulators, however, introduce amplitude modulation on the branch signals and consequently IMD in the amplifiers resulting in an unwanted residual spectrum at the output. A method is presented and used to quickly estimate LINC transmitter output spectrum as a function of the misalignments without lengthy simulations for every set of parameters. Instead, spectral properties are collected once for a given modulation scheme by means of simulation and thereafter the LINC transmitter output spectrum can be calculated directly for various sets of parameters including power amplifier nonlinearities. Numerical results show that these misalignments give residual spectrum levels on a par with those caused by the mismatch between the two branches. However, misalignments in quadrature modulators are more serious since they cannot be compensated for as easily as a mismatch between the branches. In practice, the method can be used to calculate the boundaries for the misalignments from a given spectrum mask specification and, based on this, select appropriate quadrature modulators or correction techniques for reducing the misalignments