tailieunhanh - Advanced Microwave Circuits and Systems Part 5

Tham khảo tài liệu 'advanced microwave circuits and systems part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 134 Advanced Microwave Circuits and Systems which is designed to be fully automatic has been used. This testbed is configured so that the computer plays virtually the role of the digital predistorter in the transmission chain. The chapter is organized as follows. Section 2 discusses nonlinear distortion. In section 3 we present advanced analytical development validated by simulations in Matlab. Section 4 describes the measurements test bench and experimentally evaluates the performance of the adaptive digital baseband predistortion technique. Section 5 concludes and discusses some perspectives. 2. Nonlinear Distortion In order to analyze a given system it is relevant to represent it by a mathematical model. A linear system Fig. 1 may be described either by its impulse response g t or by its frequency response G f which is the Fourier transform of g t . The output signal y t is related to the input signal x t and the impulse response g t by the convolution integral y t J x T g t T dT. Equivalently the output of the system expressed in the frequency domain is Y f X f G f . Thus a linear system modifies the spectral components of its input signal according to the special form of its frequency response G f . Modifications on the amplitude spectrum of the input signal results from a simple product Y f X f 11 G f and those on the phase spectrum from a simple addition ZY f ZX f ZG f . In order to preserve the particular form of the information-bearing signal the output signal y t could differ from the input signal only by a constant factor a and a constant delay T y t ax t t . Every linear system whose amplitude response is not constant or whose phase response is not linear may incur distortion to the input signal. This is what we call linear amplitude and linear phase distortion respectively. Time domaine _ g v _ x t Linear y t x t g t X f system Y f X f G f ịG f J Frequency domain Fig. 1. Linear system In nonlinear NL systems .