Why result of FFT command is not good?
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Hello every body. I used fft command for recognize bandwidth of a set of datas... First data pack has nearly 25000 time domain data with 25 seconds sampling time and next data pack has 12000 time domain data with 1 second sampling time, then result of fft and its absolute value are same, we know from nyquist sampling theory that by 25 seconds sampling time we cant have freqeuncy above o.o2 Hz and by 1 second sampling time we cant have freqeuncy above 0.5 Hz, but result are same similar attached picture... Red graph is 1 seconds sampling time and blue graph is 25 seconds sampling time, please care that horizontally must be multiply "0.01" and its dimension is rad/s and vertically is amplitude of signal... Please help me. Thanks alot.
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Matt J
le 31 Mar 2015
Modifié(e) : Matt J
le 31 Mar 2015
Remember that the duration of the data pack will affect frequency domain sampling according to
deltaFrequency=1/time_duration
So, your first data pack is getting its spectrum sampled at increments of .04 Hz while your second pack is being sampled at 1 Hz increments. Also keep in mind that this is not a continuous Fourier transform. To scale the FFT consistently with the continuous Fourier transform, you must multiply by the time sampling interval, which is also different for each of your two data packs.
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