OpenStax_CNX

You are here: Home » Content » Understanding Pole/Zero Plots on the Z-Plane

Recently Viewed

This feature requires Javascript to be enabled.

Tags

(What is a tag?)

These tags come from the endorsement, affiliation, and other lenses that include this content.

Understanding Pole/Zero Plots on the Z-Plane

Module by: Michael Haag. E-mail the author

Summary: This module will look at the relationships between the z-transform and the complex plane. Specifically, the creation of pole/zero plots and some of their useful properties are discussed.

Note: You are viewing an old version of this document. The latest version is available here.

Introduction to Poles and Zeros of the Z-Transform

Once the Z-transform of a system has been determined, one can use the information contained in function's polynomials to graphically represent the function and easily observe many defining characteristics. The Z-transform will have the below structure, based on Rational Functions:

Xz=PzQz Xz Pz Qz
(1)

The two polynomials, PzPz and QzQz, allow us to find the poles and zeros of the Z-Transform.

Definition 1: zeros
1. The value(s) for z where Pz=0 Pz 0.
2. The complex frequencies that make the overall gain of the filter transfer function zero.
Definition 2: poles
1. The value(s) for z where Qz=0 Qz 0.
2. The complex frequencies that make the overall gain of the filter transfer function infinite.

Example 1

Below is a simple transfer function with the poles and zeros shown below it. Hz=z+1(z12)(z+34) Hz z 1 z 1 2 z 3 4

The zeros are: 1 1

The poles are: 1234 1 2 3 4

The Z-Plane

Once the poles and zeros have been found for a given Z-Transform, they can be plotted onto the Z-Plane. The Z-plane is a complex plane with an imaginary and real axis referring to the complex-valued variable zz. The position on the complex plane is given by reiθ r θ and the angle from the positive, real axis around the plane is denoted by θθ. When mapping poles and zeros onto the plane, poles are denoted by an "x" and zeros by an "o". The below figure shows the Z-Plane, and examples of plotting zeros and poles onto the plane can be found in the following section.

Examples of Pole/Zero Plots

This section lists several examples of finding the poles and zeros of a transfer function and then plotting them onto the Z-Plane.

Example 2: Simple Pole/Zero Plot

Hz=z(z12)(z+34) Hz z z 1 2 z 3 4

The zeros are: 0 0

The poles are: 1234 1 2 3 4

Example 3: Complex Pole/Zero Plot

Hz=(zi)(z+i)(z(1212i))(z12+12i) Hz z z z 1 2 1 2 z 1 2 1 2

The zeros are: ii

The poles are: 112+12i1212i 1 1 2 1 2 1 2 1 2

MATLAB - If access to MATLAB is readily available, then you can use its functions to easily create pole/zero plots. Below is a short program that plots the poles and zeros from the above example onto the Z-Plane.



% Set up vector for zeros
z = [j ; -j];

% Set up vector for poles
p = [-1 ; .5+.5j ; .5-.5j];

figure(1);
zplane(z,p);
title('Pole/Zero Plot for Complex Pole/Zero Plot Example');



Pole/Zero Plot and Region of Convergance

The region of convergence (ROC) for XzXz in the complex Z-plane can be determined from the pole/zero plot. Although several regions of convergence may be possible, where each one corresponds to a different impulse response, there are some choices that are more practical. A ROC can be chosen to make the transfer function causal and/or stable depending on the pole/zero plot.

Filter Properties from ROC

• If the ROC extends outward from the outermost pole, then the system is causal.
• If the ROC includes the unit circle, then the system is stable.
Below is a pole/zero plot with a possible ROC of the Z-transform in the Simple Pole/Zero Plot discussed earlier. The shaded region indicates the ROC chosen for the filter. From this figure, we can see that the filter will be both causal and stable since the above listed conditions are both met.

Example 4

Hz=z(z12)(z+34) Hz z z 1 2 z 3 4

Frequency Response and the Z-Plane

The reason it is helpful to understand and create these pole/zero plots is due to their ability to help us easily design a filter. Based on the location of the poles and zeros, the magnitude response of the filter can be quickly understood. Also, by starting with the pole/zero plot, one can design a filter and obtain its transfer function very easily. Refer to this (Reference) for information on the relationship between the pole/zero plot and the frequency response.

Content actions

Give feedback:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks