I am not sure why is it happening like this
Number of cycles = 4
So, with 3150 samples per cycle, the time for one cycle is 6.35e-06 x 3150 = 20 ms. That’s an equivalent frequency of 50 Hz and, with a 4 cycle analysis, the lowest frequency is 12.5 Hz.
Should I take only one cycle?
The lowest resolution will then be 50 Hz – your choice.
I read that to analyse the data better more number of cycles are
More is usually better but, if you are happy with a base frequency of 12.5 Hz, why take more?
If your 4 cycle window occupies a time frame of 80 ms then, apart from the DC component, the minimum FFT resolution is 1/0.08 = 12.5 Hz. The maximum FFET frequency component is determined by your sampling time.
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