What is the discrete-time Fourier transform (DTFT) of the sequence given: x[n] = αnu[n], α < 1?

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  1. \(\frac{1}{{1 - \alpha {e^{ - j\omega }}}}\)
  2. \(\frac{1}{{1 - \alpha {e^{j\omega }}}}\)
  3. \(\frac{\alpha }{{1 - \alpha {e^{ - j\omega }}}}\)
  4. \(\frac{\alpha }{{1 + \alpha {e^{j\omega }}}}\)

Answer (Detailed Solution Below)

Option 1 : \(\frac{1}{{1 - \alpha {e^{ - j\omega }}}}\)

Detailed Solution

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The Discrete-Time Fourier transform of a signal of infinite duration x[n] is given by

\(X\left( \omega \right) = DTFT\left\{ {x\left[ n \right]} \right\} = \mathop \sum \limits_{n = - \infty }^\infty x\left[ n \right]{e^{ - j\omega n}}\)

Given signal is, x[n] = αn u[n]

\(X\left( \omega \right) = \mathop \sum \limits_{n = - \infty }^\infty {\alpha ^n}u\left[ n \right]{e^{ - j\omega n}}\)

\( = \mathop \sum \limits_{n = 0}^\infty {\left( {\alpha {e^{ - j\omega }}} \right)^n}\)

\( = \frac{1}{{1 - \alpha {e^{ - j\omega }}}}\)

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