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Z -transforms

Z -transforms. Z -transforms. For discrete-time systems, z -transforms play the same role as Laplace transforms do in continuous-time systems As with the Laplace transform, we compute forward and inverse z -transforms by use of transforms pairs and properties. Bilateral Forward z -transform.

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Z -transforms

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  1. Z-transforms

  2. Z-transforms • For discrete-time systems, z-transforms play the same role as Laplace transforms do in continuous-time systems • As with the Laplace transform, we compute forward and inverse z-transforms by use of transforms pairs and properties Bilateral Forward z-transform Bilateral Inverse z-transform

  3. h[n] = d[n] Region of convergence: entire z-plane h[n] = d[n-1] Region of convergence: entire z-plane except z = 0 h[n-1]  z-1 H(z) h[n] = an u[n] Region of convergence: |z| > |a| which is the complement of a disk Z-transform Pairs

  4. Region of the complex z-plane for which forward z-transform converges Four possibilities (z=0 is a special case and may or may not be included) Im{z} Im{z} Entire plane Disk Re{z} Re{z} Im{z} Im{z} Intersection of a disk and complement of a disk Complement of a disk Re{z} Re{z} Region of Convergence

  5. h[n] 5 4 3 2 1 n 1 2 3 4 5 Applying Z-transform Definition • Finite extent signal Polynomial in z-1 • Five-tap delay line Impulse response h[n] Transfer function H(z) h[n] = n ( u[n] – u[n-6] ) n = -1:6; h = n .* ( stepfun(n,0) - stepfun(n-6,0) ); stem(n, h);

  6. BIBO Stability • Rule #1: Poles inside unit circle (causal signals) • Rule #2: Unit circle in region of convergence Analogy in continuous-time: imaginary axis would be in region of convergence of Laplace transform • Example: BIBO stable if |a| < 1 by rule #1 BIBO stable if |z| > |a| includes unit circle; hence, |a| < 1 by rule #2 BIBO means Bounded-Input Bounded-Output

  7. Inverse z-transform • Definition • Yuk! Using the definition requires a contour integration in the complex z-plane Use Cauchy residue theorem (from complex analysis) OR Use transform tables and transform pairs? • Fortunately, we tend to be interested in only a few basic signals (pulse, step, etc.) Virtually all signals can be built from these basic signals For common signals, z-transform pairs have been tabulated

  8. Example • Ratio of polynomial z-domain functions • Divide through by the highest power of z • Factor denominator into first-order factors • Use partial fraction decomposition to get first-order terms

  9. Example (con’t) • Find B0 by polynomial division • Express in terms of B0 • Solve for A1 and A2

  10. Example (con’t) • Express X(z) in terms of B0, A1, and A2 • Use table to obtain inverse z-transform • With the unilateral z-transform, or the bilateral z-transform with region of convergence, the inverse z-transform is unique

  11. Z-transform Properties • Linearity • Right shift (delay) Second property used in solving difference equations Second property derived in Appendix N of course reader by decomposing the left-hand side as follows:x[n-m] u[n] = x[n-m] (u[n] – u[n-m]) + x[n-m] u[n-m]

  12. Z-transform Properties • Convolution definition • Take z-transform • Z-transform definition • Interchange summation • Substituter = n - m • Z-transform definition

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