FFT of recorded data

Hi, I am trying to calculate the FFT of a signal in order to calculate a response amplitude operator of a body in water. I have a time series of the data which is 10001 points long. From the time series the largest amplitude looks to be around 10. However, when i do a FFT of the data and find the max value i seem to get around 700? Here is what i have.

x=input_data
X=fft(x);
X_mag=abs(X);
plot(X_mag);
amplitude=(X/10001)*2;

Any help would be great!

Answer 1

[Star Strider]

The correct use of the fft function is documented in the code between the first two figures (plots) in the documentation for fft. That should give you the correct amplitudes and frequencies.

[Pete]

i have tried using the nextpow2 function as well as is done in the fft documentation however i still yield amplitudes that are way to big!

[Star Strider]

Without actually having the data (or a representative subset of it) to work with, I can’t provide an explanation for your results.