What is Aliasing
This page covers about Aliasing basics and mention Anti-Aliasing Technique.
Aliasing occurs due to inadequate sampling used in A to D conversion. Figure-1 a) depicts analog continous spectrum band limited to +B on upper side and -B on lower side. Let us understand sampling of this signal.
When this signal is sampled using Fs > 2B then it will separate spectral replications at folding frequency of about +/- fs/2. This is shown in figure-1 (b). The relationship fs >= 2B is referred as Nyquist Criteria.
When sampling frequency of fs = 1.5*B is choosen then, it will result into spectral replications. This type of sampling is known as undersampling. Here lower and upper edge of spectral replications exist around +fs and -fs. This overlaps with original sampled spectrum centered around 0 Hz. This condition is known as aliasing. This is shown in figure-1.
In practical conditions, analog signal exists with noise energy. When this signal is sampled at rate greater than 2B then it will prevent replications overlapp on each other. But resultant discrete spectrum will have noise energy between -fs/2 and +fs/2.
Hence in order to prevent noise energy, analog low pass anti alias filter is used to attenuate unwanted signal energy above +B and below -B Hz.
With the use of anti-alias filter spectral aliasing can be avoided at the output of A to D Converter. This is shown in figure-2 above.