Extracting periodicity with Fast Fourier Transform

AI Thread Summary
To extract the period from a complex discrete signal using Fast Fourier Transform, it's crucial to correctly apply the relationship between period and frequency, which are inversely related. The discussion highlights a systematic bias in the initial Matlab script, suggesting that dividing by the sample frequency is necessary. However, confusion arises regarding whether to use the y-axis or x-axis values from the frequency domain graph. Clarification on the definitions of index and sample frequency is also sought. Ultimately, the suggested formula for calculating the period is to use the length of the sequence multiplied by the time step (dt) divided by the index.
Ebert87
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Hello all,

I want to extract the period out of a complex discrete signal.

Currently I have the Matlabscript of the attachement.

However, the values I get out of this script are not correct. There is some kind of systematic bias in it.

I think it has something to do with index * samplefrequency.

Anyone an idea what could be wrong?

Thanks in advance!
 

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Right, you should divide by sample frequency. Period and frequency have inverse relation.
 
Hassan2 said:
Right, you should divide by sample frequency. Period and frequency have inverse relation.

Thanks, I tried it, but now the values are extremely big. Maybe I am using the y-axis value of the frequency domain graph, instead of the x-axis value (which says something about thefrequency of the original signal, right?)?

Getting confused now :confused:
 
What are the values of index and Samplefreq?

I think you should use this:

Period=(Length of the sequence)*dt/index
 

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