# 3.7 Understanding pole/zero plots on the z-plane

 Page 1 / 1
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.

## Introduction to poles and zeros of the z-transform

It is quite difficult to qualitatively analyze the Laplace transform and Z-transform , since mappings of their magnitude and phase or real part andimaginary part result in multiple mappings of 2-dimensional surfaces in 3-dimensional space. For this reason, it is verycommon to examine a plot of a transfer function's poles and zeros to try to gain a qualitative idea of what a system does.

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

$X(z)=\frac{P(z)}{Q(z)}$

The two polynomials, $P(z)$ and $Q(z)$ , allow us to find the poles and zeros of the Z-Transform.

zeros
The complex frequencies that make the overall gain of the filter transfer function zero.
poles
The complex frequencies that make the overall gain of the filter transfer function infinite.

Below is a simple transfer function with the poles and zeros shown below it. $H(z)=\frac{z+1}{(z-\frac{1}{2})(z+\frac{3}{4})}$

The zeros are: $\{-1\}$

The poles are: $\{\frac{1}{2}, -\left(\frac{3}{4}\right)\}$

## The z-plane

Once the poles and zeros have been found for a given Z-Transform, they can be plotted onto the Z-Plane. TheZ-plane is a complex plane with an imaginary and real axis referring to the complex-valued variable $z$ . The position on the complex plane is given by $re^{i\theta }$ and the angle from the positive, real axis around the plane is denoted by $\theta$ . When mapping poles and zerosonto the plane, poles are denoted by an "x" and zeros by an "o". The below figure shows theZ-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 theZ-Plane.

## Simple pole/zero plot

$H(z)=\frac{z}{(z-\frac{1}{2})(z+\frac{3}{4})}$

The zeros are: $\{0\}$

The poles are: $\{\frac{1}{2}, -\left(\frac{3}{4}\right)\}$

## Complex pole/zero plot

$H(z)=\frac{(z-i)(z+i)}{(z-\frac{1}{2}-\frac{1}{2}i)(z-\frac{1}{2}+\frac{1}{2}i)}$

The zeros are: $\{i, -i\}$

The poles are: $\{-1, \frac{1}{2}+\frac{1}{2}i, \frac{1}{2}-\frac{1}{2}i\}$

## Pole-zero cancellation

An easy mistake to make with regards to poles and zeros is to think that a function like $\frac{(s+3)(s-1)}{s-1}$ is the same as $s+3$ . In theory they are equivalent, as the pole and zero at $s=1$ cancel each other out in what is known as pole-zero cancellation . However, think about what may happen if this were a transfer function of a system that wascreated with physical circuits. In this case, it is very unlikely that the pole and zero would remain in exactly thesame place. A minor temperature change, for instance, could cause one of them to move just slightly. If this were tooccur a tremendous amount of volatility is created in that area, since there is a change from infinity at the pole tozero at the zero in a very small range of signals. This is generally a very bad way to try to eliminate a pole. A muchbetter way is to use control theory to move the pole to a better place.

## Repeated poles and zeros

It is possible to have more than one pole or zero at any given point. For instance, the discrete-time transfer function $H(z)=z^{2}$ will have two zeros at the origin and the continuous-time function $H(s)=\frac{1}{s^{25}}$ will have 25 poles at the origin.

MATLAB - If access to MATLAB is readily available, then you can use its functions to easily createpole/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 zerosz = [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');

## Stability and control theory

Now that we have found and plotted the poles and zeros, we must ask what it is that this plot gives us. Basically whatwe can gather from this is that the magnitude of the transfer function will be larger when it is closer to the poles andsmaller when it is closer to the zeros. This provides us with a qualitative understanding of what the system does at variousfrequencies and is crucial to the discussion of stability .

## Pole/zero plots and the region of convergence

The region of convergence (ROC) for $X(z)$ 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, thereare some choices that are more practical. A ROC can be chosen to make the transfer function causal and/or stable dependingon 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, wecan see that the filter will be both causal and stable since the above listed conditions are both met.

$H(z)=\frac{z}{(z-\frac{1}{2})(z+\frac{3}{4})}$

## Frequency response and pole/zero plots

The reason it is helpful to understand and create these pole/zero plots is due to their ability to help us easilydesign a filter. Based on the location of the poles and zeros, the magnitude response of the filter can be quicklyunderstood. Also, by starting with the pole/zero plot, one can design a filter and obtain its transfer function veryeasily.

what does nano mean?
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
for screen printed electrodes ?
SUYASH
What is lattice structure?
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
what's the easiest and fastest way to the synthesize AgNP?
China
Cied
types of nano material
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
what is nano technology
what is system testing?
preparation of nanomaterial
how to synthesize TiO2 nanoparticles by chemical methods
Zubear
what's the program
Jordan
?
Jordan
what chemical
Jordan
how did you get the value of 2000N.What calculations are needed to arrive at it
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!