3. continuous and discrete time fourier series - upt · continuous and discrete time fourier series...

1

3. Continuous and discrete time Fourier series Quick overview: Continuous time Fourier series The signal x(t) can be decomposed into a Fourier series

The Fourier transform is defined by

where x(t) is the c.t. signal. Then the coefficients of the exponential Fourier series are

Discrete time Fourier series The discrete time signal x[n] can be decomposed into a Fourier series:

The Fourier transform:

Coefficients of the Fourier series

Where kc -magnitude spectrum

{ }arg kc -phase spectrum

2kc -power spectrum

2

Continuous-time Fourier series. Problems. 1) Determine and sketch the magnitude spectra of the following

Solution.

3

4

e)

5

Homework. ( ) 2tx t e−= , for the period T=4 and ( )2, 2t∈ − . Fourier coefficients are

2) Determine the Fourier series for the sketched signals:

6

Solution. From the pairs signal – Fourier transform we have

From the properties of the exponential Fourier series

The Fourier series for the signal x(t) then becomes:

Remember that

We obtain the nth order derivative of the signal, until this is a sum of unit impulses (Dirac); we take only the restricted signal for T=4.

The 1st order derivative in discontinuity points is

. For example, at 0 we have,

7

c) The second order derivative is obtained.

8

3) a)Determine the connection between the Fourier coefficients of the input periodic signal and the output periodic signal, from a linear time-invariant system LTI. b)Determine the response of a filter (magnitude and phase spectra below) to the following signals:

Solution. a)

9

10

Homework:

Discrete-time Fourier series. Problems 1) Determine and sketch the magnitude, phase and power spectra for the following:

a) If x[n] is periodic with period N, the signal can be written as

with the Fourier coefficients

Then the signal becomes:

11

Here the period is

12

--homework