The purpose of this post is to provide detailed explanation about Aliasing phenomena and how it will reflect in the measured signal.

Aliasing – Facts:

• “Aliasing creates erroneous signal due to insufficient sampling”.
• As per Nyquist Criteria, The sample rate must be greater than or equal to two times the highest frequency component in the input signal (i.e. Fs = 2*Fmax). If this rule is violated, unwanted or undesirable signals appear in the frequency band of interest. This is called “Aliasing”.
• We could never see Aliasing in the modern vibration analyzers as they are all equipped with Anti-Aliasing filters.

Example:

Consideration:

Sampling Frequency = 16 Hz

Total no of samples = 16

That means, the maximum frequency that could be viewed properly is 8 Hz.

As per Nyquist rule, if there is any frequency component above 8 Hz (Fs/2) present in the signal, it must create erroneous signal (the signal which is not really present in the input) in the frequency band of Interest. Let’s have a step by step look with different input signals for the above sampling criteria. The amplitude of the signal is taken as 2 units.

Here, Sampling time = Total no of samples / Sampling Frequency  = 1 Second