Predistortion nonlinear compensator for microwave SSB‐AM system

Abstract

AbstractThe authors previously proposed a complex power‐series representation for input‐output nonlinearities and, on the basis of analysis of a predistortion (PD) compensation algorithm using this representation, developed a complex synthesizing PD algorithm which can compensate simultaneously for phase and amplitude nonlinearities in the TWT amplifier [1]. The algorithm is applied in this paper to compensating the nonlinearity of a TWTA operating in a high‐capacity SSB‐AM microwave system and describes the realization of a nonlinear distortion compensator which gives excellent distortion reduction with high stability. First the PD circuit required to achieve the prescribed distortion reduction and the nonlinear characteristics and degradation factors of the TWTA are analyzed and the design method for the complex synthesizing PD circuit is clarified. Second, in order to achieve easy adjustment and long‐term stability in the counterbalancing compensation, an automatic self‐adjusting stabilization method using microprocessors is proposed. Application of the self‐adjusting system to this circuit has achieved a long‐term stable reduction of more than 30 dB per 30 MHz in the nonlinear distortion noise of TWTA. This reduction is sufficient to develop a practical radio relay system.