F N: Nyquist frequency
1. Nyquist sampling theorem explains the relationship between the sample rate and the frequency of the measured signal. It describes the sampling rate fs of the signal of interest must be greater than the highest frequency component twice.
(Sampling rate to ensure that small distortion of the measured signal, although 2 octave (critical) did not lose the trust of the frequency components of the signal, but it is difficult to ensure that the amplitude of the signal without distortion)
2. When the sampling rate is less than. 2F N occur when aliasing (the presence of high-frequency signal as a low-frequency signal).
solution: can add a low-pass filter anti-aliasing filter.