Z-Plane Analysis DR. Wajiha Shah

Content Introduction z-Transform Zeros and Poles Region of Convergence Important z-Transform Pairs Inverse z-Transform z-Transform Theorems and Properties System Function

The z-Transform Introduction

Why z-Transform? A generalization of Fourier transform Why generalize it? – FT does not converge on all sequence – Notation good for analysis – Bring the power of complex variable theory deal with the discrete-time signals and systems

The z-Transform z-Transform

Definition The z-transform of sequence x(n) is defined by Let z = e j. Fourier Transform

z-Plane Re Im z = e j Fourier Transform is to evaluate z-transform on a unit circle.

z-Plane Re Im X(z)X(z) Re Im z = e j

Periodic Property of FT Re Im X(z)X(z) X(e j ) Can you say why Fourier Transform is a periodic function with period 2 ?

z-Plane Analysis Zeros and Poles

Definition Give a sequence, the set of values of z for which the z-transform converges, i.e., |X(z)|<, is called the region of convergence. ROC is centered on origin and consists of a set of rings.

Example: Region of Convergence Re Im ROC is an annual ring centered on the origin. r

Stable Systems Re Im 1 A stable system requires that its Fourier transform is uniformly convergent. Fact: Fourier transform is to evaluate z-transform on a unit circle. A stable system requires the ROC of z-transform to include the unit circle.

Example: A right sided Sequence n x(n)x(n)...

Example: A right sided Sequence For convergence of X(z), we require that

a a Example: A right sided Sequence ROC for x(n)=a n u(n) Re Im 1 a a Re Im 1 Which one is stable?

Example: A left sided Sequence n x(n)x(n)...

Example: A left sided Sequence For convergence of X(z), we require that

a a Example: A left sided Sequence ROC for x(n)= a n u( n 1) Re Im 1 a a Re Im 1 Which one is stable?

The z-Transform Region of Convergence

Example: A right sided Sequence Re Im a ROC is bounded by the pole and is the exterior of a circle.

Example: A left sided Sequence Re Im a ROC is bounded by the pole and is the interior of a circle.

Example: Sum of Two Right Sided Sequences Re Im 1/2 1/3 1/12 ROC is bounded by poles and is the exterior of a circle. ROC does not include any pole.

Example: A Two Sided Sequence Re Im 1/2 1/3 1/12 ROC is bounded by poles and is a ring. ROC does not include any pole.

Example: A Finite Sequence Re Im ROC: 0 < z < ROC does not include any pole. N-1 poles N-1 zeros Always Stable

Properties of ROC A ring or disk in the z-plane centered at the origin. The Fourier Transform of x(n) is converge absolutely iff the ROC includes the unit circle. The ROC cannot include any poles Finite Duration Sequences: The ROC is the entire z-plane except possibly z=0 or z=. Right sided sequences: The ROC extends outward from the outermost finite pole in X(z) to z=. Left sided sequences: The ROC extends inward from the innermost nonzero pole in X(z) to z=0.

More on Rational z-Transform Re Im abc Consider the rational z-transform with the pole pattern: Find the possible ROCs

More on Rational z-Transform Re Im abc Consider the rational z-transform with the pole pattern: Case 1: A right sided Sequence.

More on Rational z-Transform Re Im abc Consider the rational z-transform with the pole pattern: Case 2: A left sided Sequence.

More on Rational z-Transform Re Im abc Consider the rational z-transform with the pole pattern: Case 3: A two sided Sequence.

More on Rational z-Transform Re Im abc Consider the rational z-transform with the pole pattern: Case 4: Another two sided Sequence.

The z-Transform Important z-Transform Pairs

Z-Transform Pairs Sequencez-TransformROC All z All z except 0 (if m>0) or (if m<0)

Z-Transform Pairs Sequencez-TransformROC

The z-Transform Inverse z-Transform

The z-Transform z-Transform Theorems and Properties

Linearity Overlay of the above two ROCs

Shift

Multiplication by an Exponential Sequence

Differentiation of X(z)

Conjugation

Reversal

Real and Imaginary Parts

Initial Value Theorem

Convolution of Sequences

The z-Transform System Function

Shift-Invariant System h(n)h(n) h(n)h(n) x(n)x(n) y(n)=x(n)*h(n) X(z)X(z)Y(z)=X(z)H(z) H(z)H(z)

Shift-Invariant System H(z)H(z) H(z)H(z) X(z)X(z) Y(z)Y(z)

N th -Order Difference Equation

Representation in Factored Form Contributes poles at 0 and zeros at c r Contributes zeros at 0 and poles at d r

Stable and Causal Systems Re Im Causal Systems : ROC extends outward from the outermost pole.

Stable and Causal Systems Re Im Stable Systems : ROC includes the unit circle. 1

Example Consider the causal system characterized by Re Im 1 a

Determination of Frequency Response from pole-zero pattern A LTI system is completely characterized by its pole-zero pattern. Example: Re Im z1z1 p1p1 p2p2

Determination of Frequency Response from pole-zero pattern A LTI system is completely characterized by its pole-zero pattern. Example: Re Im z1z1 p1p1 p2p2 |H(e j )|=? H(e j )=?

Determination of Frequency Response from pole-zero pattern A LTI system is completely characterized by its pole-zero pattern. Example: Re Im z1z1 p1p1 p2p2 |H(e j )|=? H(e j )=? |H(e j )| = | H(e j ) = 1 ( )

Example Re Im a dB