2 deleted 26 characters in body edited Apr 22 '15 at 12:23 SergV 82055 silver badges99 bronze badges The size of $$y$$ is 201. So frequency bin of fft is not 1 Hz. Change code for calculating $$t$$ and $$y$$: Fs=200. F=50. t = [i*1./Fs for i in range(200)] y = sin(2*pi*t*F)  Now you have only 1 peak on 50 Hz. P.S. Plot magnitude spectrum Try to modify F (49, 49.5, 49.67). Enjoy! Welcome to DSP and new questions! The size of $$y$$ is 201. So frequency bin of fft is not 1 Hz. Change code for calculating $$t$$ and $$y$$: Fs=200. F=50. t = [i*1./Fs for i in range(200)] y = sin(2*pi*t*F)  Now you have only 1 peak on 50 Hz. P.S. Plot magnitude spectrum Try to modify F (49, 49.5, 49.67). Enjoy! Welcome to DSP and new questions! The size of $$y$$ is 201. So frequency bin of fft is not 1 Hz. Change code for calculating $$t$$: Fs=200. F=50. t = [i*1./Fs for i in range(200)]  Now you have only 1 peak on 50 Hz. P.S. Plot magnitude spectrum Try to modify F (49, 49.5, 49.67). Enjoy! Welcome to DSP and new questions! 1 answered Apr 22 '15 at 10:06 SergV 82055 silver badges99 bronze badges The size of $$y$$ is 201. So frequency bin of fft is not 1 Hz. Change code for calculating $$t$$ and $$y$$: Fs=200. F=50. t = [i*1./Fs for i in range(200)] y = sin(2*pi*t*F)  Now you have only 1 peak on 50 Hz. P.S. Plot magnitude spectrum Try to modify F (49, 49.5, 49.67). Enjoy! Welcome to DSP and new questions!