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-   -   How Fourier Expansion indicates the Amplitude w.r.t a certain frequency? (http://www.physicsforums.com/showthread.php?t=527824)

genxium Sep7-11 02:58 AM

How Fourier Expansion indicates the Amplitude w.r.t a certain frequency?
 
1. The problem statement, all variables and given/known data

I wanna know how could I extract the amplitude(of the sinusoid component) of a random continuous wave w.r.t a certain frequency response? The teacher said the Fourier Expansion can do that but I'm really confused by the limits and integrals.


2. Relevant equations

[itex]F(\omega)=\int f(t) e^{-j \omega t} dt[/itex]

3. The attempt at a solution

I tried the very easy example and wanna extract the amplitude where the frequency matches(say [itex]\omega = \omega_0 [/itex]). [itex]f(t)=A \cdot cos\omega_0t,\hat{f}(t)=A \cdot e^{j \omega_0 t},\hat{F}(\omega)=\int \hat{f}(t) \cdot e^{-j \omega t} dt [/itex], range ([itex]-\infty ,\infty [/itex]), but it turned out to be [itex]Re\{ \hat{F}( \omega ) \}=A \cdot \frac{sin(\omega_0 - \omega ) (t_2-t_1)}{\omega_0 - \omega}[/itex], where [itex]t_2=\infty,t_1=-\infty[/itex] , it's weird if I follow the basic operation of sin function, I got [itex]Re\{ \hat{F}( \omega ) \}=2 \cdot A \cdot \frac{sin(\omega_0 - \omega ) \infty}{\omega_0 - \omega}[/itex], and then although applying that [itex]lim \frac{sinx}{x} -> 1[/itex] while x->0, it's 2A, besides I don't even know if this's right.

I have no idea what happened...

Any help will be appreciated !!!

genxium Sep7-11 11:41 AM

Re: How Fourier Expansion indicates the Amplitude w.r.t a certain frequency?
 
Terribly sorry that my teacher has corrected my mistakes, the Amplitude should be [itex]|F(\omega)|[/itex] instead of [itex]Re \{ \hat{F}(\omega) \}[/itex],but the calculation becomes even harder, I'm still trying on this question.


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