# Homework 5, ECE438, Fall 2014, Prof. Boutin

Hard copy due in class, Monday October 6, 2014.

## Presentation Guidelines

• Write only on one side of the paper.
• Use a "clean" sheet of paper (e.g., not torn out of a spiral book).
• Staple the pages together.
• Include a cover page.

## Questions 1

Compute the DFT of the following signals x[n] (if possible). How does your answer relate to the Fourier series coefficients of x[n]?

a) $x_1[n] = \left\{ \begin{array}{ll} 1, & n \text{ multiple of } N\\ 0, & \text{ else}. \end{array} \right.$

b) $x_1[n]= e^{j \frac{2}{3} \pi n};$

c) $x_5[n]= e^{-j \frac{2}{1000} \pi n};$

d) $x_2[n]= e^{j \frac{2}{\sqrt{3}} \pi n};$

e) $x_6[n]= \cos\left( \frac{2}{1000} \pi n\right) ;$

f) $x_2[n]= e^{j \frac{\pi}{3} n } \cos ( \frac{\pi}{6} n )$

g) $x_8[n]= (-j)^n .$

h) $x_3[n] =(\frac{1}{\sqrt{2}}+j \frac{1}{\sqrt{2}})^n$

Note: All of these DFTs are VERY simple to compute. If your computation looks like a monster, look for a simpler approach!

## Question 2

Compute the inverse DFT of $X[k]= e^{j \pi k }+e^{-j \frac{\pi}{2} k}$.

Note: Again, this is a VERY simple problem. Have pity for your grader, and try to use a simple approach!

## Question 3

Prove the time shifting property of the DFT.

## Discussion

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