matlab – Reason for frequency steps shown in Simulink FFT analysis tool

I am not sure why is it happening like this

Information: –

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
required

More is usually better but, if you are happy with a base frequency of 12.5 Hz, why take more?

Summary

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.