Post on 02-Jan-2016
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Lecture 2: Z-TransformXILIANG LUO
2014/9
Fourier Transform
Convergence A sufficient condition: absolutely summable
it can be shown the DTFT of absolutely summable sequence converge uniformly to a continuous function
Square Summable A sequence is square summable if:
For square summable sequence, we have mean-square convergence:
โ๐=โโ
โ
|๐ฅ [๐ ]|2<โ
Z-Transform
a function of the complex variable: z
If we replace the complex variable z by , we have the Fourier Transform!
Z-Transform & Fourier Transform
Complex z-plane
Region of Convergence The set of z for which the z-transform converges is called ROC of the z-transform.
Absolutely summable criterion:
ROC ROC consists of a ring in the z-plane
Closed-Form in ROC When X(z) is a rational function inside ROC, i.e.
P(z), Q(z) are polynomials in z Zeros: values of z such that X(z) = 0 Poles: values of z such that X(z) = infinity
Z-Transform Example: Right-Sided
Z-Transform Example:Left-Sided
Diff. Sum, Same Z-Transform? One is right-sided exponential sequence
One is left-sided exponential sequence
But they share the same algebraic expressions for their Z-Transforms
This emphasizes the importance of the region of convergence!!
ROC Properties
ROC Properties
ROC Properties
Inverse z-Transform From the z-Transform, we can recover the original sequence using the following complex contour integral:
๐ฅ [๐ ]= 12๐ ๐โฎ๐ถ
โ
๐ (๐ง )๐ง๐โ1๐๐ง
C is a closed contour within the ROC of the z-transform
Inverse z-Transform Methods Inspection
familiar with the common transform pairs
Partial Fraction Expansion
Power Series Expansion
z-Transform Properties 1. Linearity
2. Time Shifting
3. Multiplication by an Exponential Sequence
z-Transform Properties 4. Differentiation of X(z)
5. Conjugation of a Complex Sequence
7. Time Reversal
z-Transform Properties 7. Convolution of Sequences
z-Transform and LTI Systems LTI system is characterized by its impulse response h[n]
h[n]x[n] y[n]
๐ฆ [๐ ]=๐ฅ [๐ ]โh[๐]
๐ (๐ง )=๐ (๐ง )ร๐ป (๐ง )
H(z) is called the system function of this LTI system!
Cauchy-Riemann Equations If function f(z) is differentiable at z0=x0+y0, then its component functions must satisfy the following conditions:
๐ (๐ง )=๐ข (๐ฅ , ๐ฆ )+๐๐ฃ (๐ฅ , ๐ฆ)
๐๐ข๐๐ฅ
=๐ ๐ฃ๐ ๐ฆ
๐๐ข๐ ๐ฆ
=โ๐ ๐ฃ๐ ๐ฅ
Analytic Functions A function f(z) is analytic at a point z0 if it has a derivative at each point in some neighborhood of z0.
So, If f(z) is analytic at a point z0, it must be analytic at each point in some neighborhood of z0.
Taylor Series Theorem: Suppose that a function f is analytic throughout a disk: |z-z0|<R0, centered at z0 and with radius R0, then f(z) has the power series representation:
๐ (๐ง )=โ๐=0
+โ
๐๐ (๐ง โ๐ง0 )๐ |๐งโ ๐ง0|<๐ 0
๐๐=๐ (๐ )(๐ง0)๐ !
Laurent Series If a function is not analytic at a point z0, one cannot apply Taylorโs theorem at that point!
Laurentโs Theorem: Suppose a function f is analytic throughout an annular domain centered at z0:
Let C denote any positively oriented simple closed contour around z0 and lying in the domain, then, at each point in the domain, f(z) has the series representation:
๐ 1<|๐งโ๐ง 0|<๐ 2
๐ (๐ง )= โ๐=โโ
+โ
๐๐ (๐งโ๐ง 0 )๐
Laurent Series
๐๐=12๐ ๐โฎ๐ถ
โ ๐ (๐ง )๐๐ง
(๐งโ ๐ง0 )๐+1
๐ (๐ง )= โ๐=โโ
+โ
๐๐ (๐งโ๐ง 0 )๐
Homework Problems
3.59:
3.57:
3.52:
3.56:
Next Sampling of Continuous-Time Signals
Please read the textbook Chapter 4 in advance!