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Chapter: Signals and Systems - Classification of Signals and Systems

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Continuous Time Fourier Transform

Fourier expansion coefficient x[k]:

Continuous Time Fourier Transform:

 

The Fourier expansion coefficient x[k] ( in ak  OWN) of a periodic signal is  



and the Fourier expansion of the signal is:


which can also be written as:



When the period of xT(t) approaches infinity, T->(infinity) the periodic signal xT(t) becomes a non-periodic signal and the following will result:

 

Interval between two neighboring frequency components becomes zero:


Discrete frequency becomes continuous frequency:


 

Summation of the Fourier expansion in equation (a) becomes an integral:


the second equal sign is due to the general fact:


 

Time integral over in equation (b) becomes over the entire time axis:


In summary, when the signal is non-periodic x(t)=limT->infxT(t)  the Fourier expansion becomes Fourier transform. The forward transform (analysis) is:


and the inverse transform (synthesis) is:


Comparing Fourier coefficient of a periodic signal  xT(t) withwith Fourier spectrum of a non-periodic signal x(t)


we see that the dimension of X(ω) is different from that of     X[k]:


x(t) distributed along the frequency axis. We can only speak of the energy contained in a particular frequency band ω1< ω< ω2

 

 

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