tailieunhanh - ebook signalsanhsystemsp2

| Chapter 4 The Z-Transform and Discrete-Time LTI Systems INTRODUCTION In Chap. 3 we introduced the Laplace transform. In this chapter we present the z-transform which is the discrete-time counterpart of the Laplace transform. The z-trans-form is introduced to represent discrete-time signals or sequences in the z-domain z is a complex variable and the concept of the system function for a discrete-time LTI system will be described. The Laplace transform converts integrodifferential equations into algebraic equations. In a similar manner the z-transform converts difference equations into algebraic equations thereby simplifying the analysis of discrete-time systems. The properties of the z-transform closely parallel those of the Laplace transform. However we will see some important distinctions between the z-transform and the Laplace transform. THE Z-TRANSFORM In Sec. we saw that for a discrete-time LTI system with impulse response h n the output y 2 of the system to the complex exponential input of the form z is y n T z H z z where 7 z Ẽ h n z n -00 A. Definition The function H z in Eq. is referred to as the z-transform of i n . For a general discrete-time signal x z the z-transform x z is defined as 00 x z - E x n z n -00 The variable z is generally complex-valued and is expressed in polar form as z rejii where r is the magnitude of z and if is the angle of z. The z-transform defined in Eq. is often called the bilateral or two-sided z-transform in contrast to the unilateral or 165 166 THE Z-TRANSFORM AND DISCRETE-TIME LTI SYSTEMS CHAP. 4 one-sided z-transform which is defined as xw Ejr n z n 0 Clearly the bilateral and unilateral z-transforms are equivalent only if x n 0 for n 0. The unilateral z-transform is discussed in Sec. . We will omit the word bilateral except where it is needed to avoid ambiguity. As in the case of the Laplace transform Eq. is sometimes considered an operator that transforms a sequence x zi into a

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