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# Exercises

Module by: Anders Gjendemsjø. E-mail the author

Summary: >Exercises to TTT4110: Information and Signal Theory, Signals

Problems related to the Signals chapter.

## Exercise 1

Find the digital frequency of xn=cos2π×3n x n 2 3 n . Is the signal periodic? If so, find the shortest possible period.

### Solution

Write cos2π×3n 2 3 n as cos2πfn 2 f n , where ff is the digital frequency. We see that the digital frequency is 3 3 . For a trigonometric signal to be periodic the digital frequency has to be a rational number, i.e f=mN f m N , where both m,N are integers. N is the signal period. Here the digital frequency is not a rational number, hence the signal is not periodic.

## Exercise 2

Find the digital frequency of xn=cos2π×4n x n 2 4 n . Is the signal periodic? If so, find the shortest possible period.

### Solution

Write cos2π×4n 2 4 n as cos2πfn 2 f n , where ff is the digital frequency. We see that the digital frequency is 4=2 4 2 . For a trigonometric signal to be periodic the digital frequency has to be a rational number, i.e f=mN f m N , where both m,N are integers. N is the signal period. In this case the digital frequency is a rational number, f=21 f 2 1 , hence the signal is periodic. The period, N, is given by N=mf=m2 N m f m 2 . Since N has to be an integer, we obtain the shortest possible period letting m=2 m 2 , which yields N=1 N 1 .

## Exercise 3

Find the digital frequency of xn=sin2π1.5n x n 2 1.5 n . Is the signal periodic? If so, find the shortest possible period.

### Solution

Write sin2π1.5n 2 1.5 n as sin2πfn 2 f n , where ff is the digital frequency. We see that the digital frequency is 1.5. The digital frequency is a rational number(3/2), hence the signal is periodic. The period, N, is given by N=mf=2m3 N m f 2 m 3 . Since N has to be an integer, we obtain the shortest possible period letting m=3 m 3 , which yields N=2 N 2 .

## Exercise 4

Referring to example 2 find the analog and digital frequency of x1t x1 t and x2n x2 n respectively.

### Solution

Using the same reasoning as above we easily see that the analog sine has frequency 1, while the discrete time sine has digital frequency 1/20.

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