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# New Module Object Example (Derived copy of Fourier Series in a Nutshell)

Module by: Chinwei Hu. E-mail the author

Summary: [abstract] This module will give a brief over of the key concepts involving the Fourier series and the tools used to decompose and approximate a given signal.

## [Introduction]

The convolution integral is the fundamental expression relating the input and output of an LTI system. However, it has three shortcomings:

1. It can be tedious to calculate.
2. It offers only limited physical interpretation of what the system is actually doing.
3. It gives little insight on how to design systems to accomplish certain tasks.
The Fourier Series, along with the Fourier Transform and Laplace Transofrm, provides a way to address these three points. Central to all of these methods is the concept of an eigenfunction (or eigenvector). We will look at how we can rewrite any given signal, ft f t , in terms of complex exponentials.

In fact, by making our notions of signals and linear systems more mathematically abstract, we will be able to draw enlightening parallels between signals and systems and linear algebra.

## [Theory] Eigenfunctions and LTI Systems

The action of a LTI system on one of its eigenfunctions est s t is

1. extremely easy (and fast) to calculate
est=Hsest s t H s s t
(1)
2. easy to interpret: just scales est s t , keeping its frequency constant.
If only every function were an eigenfunction of ...

### LTI System

... of course, not every function can be, but for LTI systems, their eigenfunctions span the space of periodic functions, meaning that for (almost) any periodic function ft f t we can find c n c n where nZ n and c i C c i such that:

ft= n = c n ei ω 0 nt f t n c n ω 0 n t
(2)
Given Equation 2, we can rewrite ft=yt f t y t as the following system

where we have: ft==n c n ei ω 0 nt f t n c n ω 0 n t yt=ft=n c n Hi ω 0 ntei ω 0 nt y t f t n c n H ω 0 n t ω 0 n t This transformation from ft f t to yt y t can also be illustrated through the process below. Note that each arrow indicates an operation on our signal or coefficients.

ft c n c n Hi ω 0 nyt f t c n c n H ω 0 n y t
(3)
where the three steps (arrows) in the above illustration represent the following three operations:
1. Transform with analysis ( Fourier Coefficient equation): c n =1T0Tfte(i ω 0 nt)d t c n 1 T t T 0 f t ω 0 n t
2. Action of on the Fourier series - equals a multiplication by Hi ω 0 n H ω 0 n
3. Translate back to old basis - inverse transform using our synthesis equation from the Fourier series: yt= n = c n ei ω 0 nt y t n c n ω 0 n t

## [Application] Physical Interpretation of Fourier Series

The Fourier series c n c n of a signal ft f t , defined in Equation 2, also has a very important physical interpretation. Coefficient c n c n tells us "how much" of frequency ω 0 n ω 0 n is in the signal.

Signals that vary slowly over time - smooth signals - have large c n c n for small nn.

Signals that vary quickly with time - edgy or noisy signals - will have large c n c n for large nn.

## [Examples]

### Example 1: Periodic Pulse

We have the following pulse function, ft f t , over the interval T2 T2 T 2 T 2 :

Using our formula for the Fourier coefficients,

c n =1T0Tfte(i ω 0 nt)d t c n 1 T t T 0 f t ω 0 n t
(4)
we can easily calculate our c n c n . We will leave the calculation as an exercise for you! After solving the the equation for our ft f t , you will get the following results:
c n ={2 T 1 T  if  n=02sin ω 0 n T 1 nπ  if  n0 c n 2 T 1 T n 0 2 ω 0 n T 1 n n 0
(5)
For T 1 =T8 T 1 T 8 , see the figure below for our results:

Our signal ft f t is flat except for two edges (discontinuities). Because of this, c n c n around n=0 n 0 are large and c n c n gets smaller as nn approaches infinity.

#### Question:

Why does c n =0 c n 0 for n=-44816 n -4 4 8 16 ? (What part of e(i ω 0 nt) ω 0 n t lies over the pulse for these values of nn?)

## [Lab]

Numerical Evaluation of Fourier Series http://www.rose-hulman.edu/DSPFirst/visible3/chapters/10confrq/labsLV/lab13/lab13.pdf

## [Summary]

Fourier analysis is very useful!

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