• Sampling theorem states that for proper reconstruction, the signal should be sampled with sampling frequency, ts, that is greater than or equal to twice, the highest modulating freq..fm,

fs $\geq$ 2fm

If this Nyquist criteria is not satisfied aliasing takes place. i.e. folder of high frequency over low frequency. To avoid aliasing, fs should be slightly greater than 2fm and / or The s/g should be strictly band limited to fm. To do so, a low pass anti-aliasing filter is used. Anti-aliasing filter is placed before sampler and it ensures that no frequency component is beyond fm.