Summary: This module brings out a clear understanding of the aliasing effect when signals are not sampled at the Nyquist Sampling Frequency.
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
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]
Sample mode: Discrete
Output Complexity :Real
Computation method:Trigonometric fcn
Samples per frame:1
2.The FFT block:
3.The Spectrum scope block:
Buffer overlap : 64
Window type: Hann
Window sampling: periodic
Specify FFT length:
No.of spectral averages:2
The Frequency Spectrum indicating 3500Hz signal posing as 500Hz due to aliasing