Revision as of 17:07, 12 September 2013

Practice Problem on Discrete-time Fourier transform computation

Compute the discrete-time Fourier transform of the following signal:

$ x[n]= u[n+2]-u[n-1] $

See these Signal Definitions if you do not know what is the step function "u[n]".

(Write enough intermediate steps to fully justify your answer.)

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Answer 1

$ x[n] = u[n+2]-u[n-1] $.

$ X_(\omega) = \sum_{n=-\infty}^{+\infty} x[n] e^{-j\omega n} $

$ = \sum_{n=-2}^{0} x[n] e^{-j\omega n} $

$ = 1+ e^{j\omega} + e^{2j\omega} $

Answer 2

$ X_{2\pi}(\omega) = \sum_{n=-\infty}^{+\infty} x[n] e^{-j\omega n} $

$ = \sum_{n=-2}^{0} x[n] e^{-j\omega n} $

$ = e^{2j\omega} + e^{j\omega} + 1 $

Answer 3

$ X_{2\pi}(\e^{omega}) = \sum_{n=-\infty}^{+\infty} e^{-j\omega n} $

$ X_{2\pi}(\e^{\omega}) = \sum_{n=-\infty}^{+\infty} e^{-j\omega n} $

$ = \sum_{n=-2}^{+\infty} u[n+2] e^{-j\omega n} - \sum_{n=1}^{+\infty} u[n-1] e^{-j\omega n} $

$ = \sum_{n=-2}^{0} (u[n+2] -u[n-1]) e^{-j\omega n} $

$ = e^{2j\omega} + e^{j\omega} + 1 $

Answer 4

$ x[n] = u[n+2]-u[n-1] $

$ x[n] = ((\delta[-2]) +(\delta[-1]) +(\delta[0])) $

so $ X[Z] = e^(2j(\omega)) +e^(j(\omega)) +1 $

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