Aliasing Distortion

A signal composed of components of different frequencies ,when sampled at a sampling frequency less than twice the value of the highest frequency component( the Nyquist sampling frequency)present in the sampled signal, results in the under sampled component posing itself as a low frequency component. This component passes through the system as an alias causing distortion. This is known as aliasing distortion.

Illustration of aliasing using Matlab/Simulink:

Simulink Block Diagram:
    Total simulation time: 0.05seconds
    

Figure 1

The Sine wave consists of components of 250Hz,1000Hz,3500Hz

This wave is sampled at 4000Hz .The component which is under sampled is 3500Hz.

This component passes through posing as a low freq component of 500Hz

Simulink model :

The Sine Wave block is picked from the Signal Processing Sources Blockset , using the Simulink Library Browser .

1.The Sine wave block is given the following block parameter values :

Amplitude:[1 4 5]

Frequency(Hz): [250 1000 3500]

Phase offset(radians):0

Sample mode: Discrete

Output Complexity :Real

Computation method:Trigonometric fcn

Sample time:1/4000

Samples per frame:1

2.The FFT block:

Default parameters

3.The Spectrum scope block:

Buffer Input

Buffer size:128

Buffer overlap : 64

Window type: Hann

Window sampling: periodic

Specify FFT length:

FFT length:128

No.of spectral averages:2

The Frequency Spectrum indicating 3500Hz signal posing as 500Hz due to aliasing

Figure 2