• The sampling theorem for low pass band limited signal states that the proper reconstruction of a sampled signal is possible if the analog signal is sampled at the sampling frequency, fs, at-least twice the highest modulating frequency (2 x fm)

i.e. fs $\geq$ 2fm

If sampling frequency is less than twice mod. frequency (fm) or if the modulating s/g is not purely band-limited to fm, then fold over of frequency takes place called aliasing.

If over sampling is performed, it leads to guard band between the 2 replicas.

Anti-aliasing filter or fs > 2fm, avoids Aliasing.