This applet draws the positive part of a fourier trasform of a cosine with frequency that you specify. In the above coordinates system you can see the function in the time domain and under that, the amplitude of the signal in the frequency domain. In the second field you put the sampling frequency. In the third field you specify the sampling period. In the fourth field you select the fft size.
Spectral resolution gets better when fft size increases.
In the fifth field you select the algorithm. Current choice is rectangle. I will add Bartlett, Hamming, Hanning and Blackman windows later. In the seventh field you can see the time elapsed in milliseconds for the FFT or DFT calculations only.