FFT
in [Matlab]
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From: Ken B on 7 May 2010 11:15 New Matlab user here. I have been given some data and asked to do a couple of things. First, was to take a FFT to find the frequency of an oscillation that should be present. The data is amplitude data as a laser was swept from frequency A to frequency B with step size N. the amplitude was recorded at every N. So I have a 1xN vector. For laser jocks out there, I know that we should be able to see a frequency component corresponding to the FSR (free spectral range) When looking in the Matlab help under FFT, it says I need to know the fs, or samples per unit time. How do I find/calculate this number?
From: Royi Avital on 7 May 2010 15:25 "Ken B" <fiziksgeek(a)gmail.com> wrote in message <hs1aqc$k5d$1(a)fred.mathworks.com>... > New Matlab user here. I have been given some data and asked to do a couple of things. First, was to take a FFT to find the frequency of an oscillation that should be present. The data is amplitude data as a laser was swept from frequency A to frequency B with step size N. the amplitude was recorded at every N. So I have a 1xN vector. > > For laser jocks out there, I know that we should be able to see a frequency component corresponding to the FSR (free spectral range) > > When looking in the Matlab help under FFT, it says I need to know the fs, or samples per unit time. How do I find/calculate this number? Are you asking for the frequency each bin in the FFT vector represents? If you have N samples sampled at Fs each bin is [num of Bin] * (Fs / N).
From: Ken B on 8 May 2010 10:43 "Royi Avital" <RoyiREMOVEAvital(a)yahoo.com> wrote in message > Are you asking for the frequency each bin in the FFT vector represents? > If you have N samples sampled at Fs each bin is [num of Bin] * (Fs / N). Not exactly, I think. I have a plot of amplitude vs. frequency with ~6000 points with a known spacing in frequency. Its sinusoidal in shape with some noise of course. I want to do a Fourier transform on it because the frequency of this sinusoidal wave should correspond to the free spectral range of a cavity in the system. All the examples I have seen, use data that is a time vs amplitude signal. I have a time vs. frequency, but want to know the frequency of the frequency signal...if that makes sense...?
From: Godzilla on 8 May 2010 14:05 "Ken B" <fiziksgeek(a)gmail.com> wrote in message <hs3t9p$n6n$1(a)fred.mathworks.com>... > "Royi Avital" <RoyiREMOVEAvital(a)yahoo.com> wrote in message > > > Are you asking for the frequency each bin in the FFT vector represents? > > If you have N samples sampled at Fs each bin is [num of Bin] * (Fs / N). > > Not exactly, I think. I have a plot of amplitude vs. frequency with ~6000 points with a known spacing in frequency. Its sinusoidal in shape with some noise of course. I want to do a Fourier transform on it because the frequency of this sinusoidal wave should correspond to the free spectral range of a cavity in the system. > > All the examples I have seen, use data that is a time vs amplitude signal. I have a time vs. frequency, but want to know the frequency of the frequency signal...if that makes sense...? Why use an FFT? I would just fit a sinusoid to the data.
From: Ken B on 10 May 2010 09:11
"Godzilla " <godzilla(a)tokyo.edu> wrote in message <hs4954$iob$1(a)fred.mathworks.com>... > "Ken B" <fiziksgeek(a)gmail.com> wrote in message <hs3t9p$n6n$1(a)fred.mathworks.com>... > > "Royi Avital" <RoyiREMOVEAvital(a)yahoo.com> wrote in message > > > > > Are you asking for the frequency each bin in the FFT vector represents? > > > If you have N samples sampled at Fs each bin is [num of Bin] * (Fs / N). > > > > Not exactly, I think. I have a plot of amplitude vs. frequency with ~6000 points with a known spacing in frequency. Its sinusoidal in shape with some noise of course. I want to do a Fourier transform on it because the frequency of this sinusoidal wave should correspond to the free spectral range of a cavity in the system. > > > > All the examples I have seen, use data that is a time vs amplitude signal. I have a time vs. frequency, but want to know the frequency of the frequency signal...if that makes sense...? > > Why use an FFT? > > I would just fit a sinusoid to the data. That would work for the main frequency dependency. There is also some higher frequency modulation visible on top of the main sinusoidal signal, I think an FFT would give me all the info at once. Plus is really bugs me that I cannot figure out how to do the FFT...haha |