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Revision as of 12:37, 18 September 2014
Fourier Transform as a Function of Frequency w Versus Frequency f (in Hertz)
A slecture by ECE student Randall Cochran
Partly based on the ECE438 Fall 2014 lecture material of Prof. Mireille Boutin.
To show the relationship between the Fourier Transform of frequency $ \omega $ versus frequency $ f $ (in hertz) we start with the definitions: $ X(w)=\int\limits_{-\infty}^{\infty} x(t)e^{-jwt} dt \qquad \qquad \qquad \qquad X(f)=\int\limits_{-\infty}^{\infty}x(t)e^{-j2\pi ft} dt $
now we let $ \omega = 2\pi f $
$ X(2\pi f)=\int\limits_{-\infty}^{\infty} x(t)e^{-j2\pi ft} dt $
making $ X(2\pi f) = X(f) $
Examples of the relationship can be shown by starting with known CTFT pairs:
Example 1. $ x(t)= e^{j\omega_o t} \qquad \qquad X(\omega ) = 2\pi \delta (\omega - \omega_o ) $
Again we will let $ \omega = 2\pi f $ in our Fourier Transform $ X(f) $
$ X(2\pi f) = 2\pi \delta (2\pi f - 2\pi f_o ) $
$ \begin{align} \\ f(x) &= \int\limits_{0}^{2\pi} sin^2(\theta) \ d\theta \\ &= \int\limits_{0}^{2\pi} \big(1-cos^2(\theta) \big)\ d\theta \\ &= \int\limits_{0}^{2\pi} \bigg( 1-\Big(\frac{1}{2} + \frac{1}{2} cos(2\theta)\Big) \bigg)\ d\theta \\ &= \int\limits_{0}^{2\pi} \frac{1}{2}\ d\theta - \frac{1}{2} \int\limits_{0}^{2\pi} cos(2\theta) \ d\theta \\ &= \pi \end{align} $
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