Code to Set the Number of Spectral Lines of an FFT(Freq Resolution)

Hi

I'm quite new to MATLAB. I'm working on spectral analysis of a signal and trying to obtain its FFT. But the problem is that my spectral lines are coming out to be quite braod and eventually, data analysis is not possible. I'm using the following code for the obtaining the FFT:

M = 2.5e-4; Fs = 1/M; T = 1/Fs; L=1000000; NFFT = 2^nextpow2(L); Y = fft(ias, NFFT)/L; f = Fs/2*linspace(0,1,NFFT/2+1); plot(f,2*abs(Y(1:NFFT/2+1)))

https://docs.google.com/leaf?id=0B4dPcFFHHl9VZGM3ZmJiMWItYzAyMy00YjAxLWFhOWUtYTA3YjRiNTI3ODE4&hl=en_US Kindly suggest some solution for this problem.

Thanks and Regards.

Answer 1

[Honglei Chen]

Hi Anshul,

I think there are two things you need to consider:

1. To get the perfect spectral line for your 60 Hz signal, you need to have a frequency sample at exact 60 Hz. Unfortunately, 60 Hz is not a multiple of your frequency resolution, i.e., Fs/NFFT, so you will not get the perfect spectral line in your spectrum.

2. Even if the condition in 1 is satisfied, if you use plot(), MATLAB automatically do the interpolation between points. So you will still see a triangle at 60 Hz. If you really really want to just see a line, you can use stem() instead.

HTH.

[Anshul]

Hi, Thanks for the reply, but i tried to use stem() and it did not work. isn't there any other way?

[Honglei Chen]

What do you mean by "it did not work"? You get an error or you don't see the perfect line? Does your code satisfy the first condition? Try to set NFFT=L and you should be able to see the spectral line. Of course I'm assuming you have a perfect 60 Hz signal ias, e.g., ias = sin(2*pi*60*(0:L-1)*T)