Dear Akram,

The Descartes Fourier transform DFT is the the Fourier transform of discrete time and so, DFT is the time discretization of the continuous time Fourier transform. The fast Fourier transform is DFT when the number of samples = 2^m with m is an integer number. If you have a Simulink model model of the signal you need onl to analyse it in the frequency domain by using FFT spectrum analyser. which is available like the time scope. What is important is to use the appropriate sampling frequency to resolve the highest possible harmonic in the signal. If you want to locate the 9th harmonic of 50 HZ you have to make fs>= 900 Hz. In order to resolve the peal locations more precisely you have even to increase fs. It may be practically good to make it 10x900= 9 kHz.

Best wishes

Thanks a lot .. let me be more specific ..

well if I have a network which has a signal with harmonics ..

and I want to see the equation of each harmonic with its phase shift .. expressed in FFT form ..

and I can specify the harmonics .. for example: I have a sine wave with 5th -7th-11th-13th harmonic and more ..

but I wanna see the fundamental equation with these harmonic only from the signal I have ..

I don't know if it is possible ..