Fourier Series

Introduction

This document takes a look at different ways of representing real periodic signals using the Fourier series. It will provide translation tables among the different representations as well as example problems using Fourier series to solve a mechanical system and an electrical system, respectively.

\section{Synthesis Equations} There are three primary Fourier series representations of a periodic signal $f(t)$ with period $T$ and fundamental frequency $\omega_0=\frac{2\pi}{T}$: \begin{align*} \mbox{Trigonometric Series}&~ & f(t)&=C_0+ \sum_{n=1}^{\infty}\left(A_n~\cos(n\omega_0 t) + B_n~\sin(n\omega_0 t)\right)\\ \mbox{Cosine Series} &~ & f(t)&= C_0 + \sum_{n=1}^{\infty}C_n~\cos(n\omega_0 t+\phi_n)\\ \mbox{Exponential Series} &~ & f(t)&= \sum_{k=-\infty}^{\infty}a_k~e^{jk\omega_0 t} \end{align*} In the series above, $A_n$, $B_n$, $C_0$, $C_n$, and $\phi_n$ are real numbers while $a_k$ may be complex. Also note that the index $n$ is used for summations between 1 and $\infty$ while $k$ is used for the summation between $-\infty$ to $\infty$ primarily to demonstrate the different ranges of the summations.

\section{Analysis Equations} The formulas for obtaining the Fourier series coefficients are: \begin{align*} A_n&=\frac{2}{T}\int_{T}f(t)~\cos(n\omega_0t)~dt & B_n&=\frac{2}{T}\int_{T}f(t)~\sin(n\omega_0t)~dt \\ C_0&=\frac{1}{T}\int_{T}f(t)~dt & C_n&= \sqrt{A_n^2+B_n^2} \\ \phi_n&=-\tan^{-1}\left(\frac{B_n}{A_n}\right)\\ a_k&=\frac{1}{T}\int_Tf(t)~e^{-jk\omega_0t}~dt & \end{align*}

\section{Translation Table} The table below summarizes how to get one set of Fourier Series coefficients from any other representation. Note that it is assumed the function being represented is real - meaning $a_n$=$a_{-n}^*$. Also, $n>0$ in the table. The core equations at use in the translation table are: \begin{align*} e^{j\theta}&=\cos(\theta)+j\sin(\theta)\\ cos(\theta+\phi)&=\cos(\theta)\cos(\phi)-\sin(\theta)\sin(\phi) \end{align*}

\renewcommand{\arraystretch}{1.5} \begin{align*} \begin{array}{|c|c|c|c|} \hline \mbox{To Get} & \multicolumn{3}{|c|}{\mbox{From}}\\ ~ & \mbox{Trig. Series} & \mbox{Cosine Series} & \mbox{Exp. Series} \\ \hline A_n & A_n & C_n\cos(\phi_n) & a_n+a_{-n}=2\Re\{a_n\}\\ \hline B_n & B_n & -C_n\sin(\phi_n) & j\left(a_n-a_{-n}\right)=-2\Im\{a_n\}\\ \hline C_0 & C_0 & C_0 & a_0 \\ \hline C_n & \sqrt{A_n^2+B_n^2} & C_n & |a_n|+|a_{-n}|=2|a_n|\\ \hline \phi_n & -\tan^{-1}\left(\frac{B_n}{A_n}\right) & \phi_n & \angle a_n\\ \hline a_0 & C_0 & C_0 & a_0 \\ \hline a_n & \frac{A_n}{2}+\frac{B_n}{2j}= \frac{A_n}{2}-j\frac{B_n}{2} & \frac{C_n}{2}\angle \phi_n & a_n\\ \hline a_{-n} & \frac{A_n}{2}-\frac{B_n}{2j}= \frac{A_n}{2}+j\frac{B_n}{2} & \frac{C_n}{2}\angle -\phi_n &a_{-n} \\ \hline \end{array} \end{align*}