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Unless you use an adequate windowing function for sampling then Coherent sampling is needed to increase your FFT spectral resolution.
Without windowing or coherent sampling you'll have waveform discontinuancies and see spectral leakage / artifacts in your FFT data i.e You see spreading of energy for one frequency across adjacent bins.
For coherent sampling, you are fitting an integer number of samples within a sampling window. If you don't use an integer number of samples the windowing function spectrum is offset from your input signal spectrum and your FFT lines are offset from their actual locations. This shift also causes side lobe artifacts etc.
For windowing you have several options such as:
Rectangular/No window / Coherent Sampling
Depending on the location of the frequency components in the signal, one window is better for your application then the other.