SUMMARYThe authors previously proposed a complex power-series representation for inputoutput 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. . IntroductionA single-sideband amplitude-modulation (SSB-AM) system can increase the transmission capacity 2 ~ 3 times compared with an FM system and is extremely effective in 57 improving the utilization efficiency of the microwave band and reducing the transmission link cost [2,3]. One of the most important problems in realizing this high capacity SSB-AM microwave system is how to reduce the distortion noise of the traveling-wave tube amplifier, the major source of nonlinear distortion.For nonlinear compensation in microwave amplifiers such as the TWTA, the feedforward method [4] and predistortion (PD) method [5,6,7] have been proposed, but in principle these methods are of the cancellation type of distortion compensation. Hence, there are some problems that complicate the construction and adjustment and excellent improvement over a wide bandwidth cannot be obtained.For the purpose of simplifying the PD method the authors previously proposed a complex power-series representation based on Volterra series expansion as a method of analyzing the amplitude and phase nonlinearities of a microwave amplifier without separating them [1].The present paper applies this complex synthesizing PD method to compensation of the nonlinear distortion of a TWTA in a 6-GHz highcapacity SSB-AM system and discusses the relation between the distortion reduction and the required characteristics of the PD circuit as well as the stabilization of compensation characteris tics in the counterbalancing type compensator.First, the PD circuit characteristics required to achieve prescribed distortion reduction and the method of constructing the 3rd-order distorter, the most important ISSN0424-8368/83/0005-0057$7.50/0 Q 1984 Scripta Publishing Co.
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