SEC. 
5 SEC. 
RASC CALA DEANS 
TRE ATA ER 
Fig. 2.8. Aliasing harmonic curves of frequency 1/5, 4/5, 6/5; 
9/5, 11/5; .. . can pass through the same 
sampling points at At = 1. 
Table 2.1. Aliasing frequencies. 
Equation 2.61 or 2.61” shows that, in cases of discrete process analysis of the 
spectrum ordinate at frequency w,, the power at higher frequencies 27 + wp, 
AG GES (a on doo\ote is spuriously folded on the real power at w, as in Fig. 2.7. 
Accordingly, when a continuous process is to be transformed into a discrete process 
sampled at time intervals Ar, aliasing is the most important effect to be considered. The 
analyzed range of the frequency spectrum is between the Nyquist frequencies 
-= and = (when Ar = 1, as in preceding sections —z to m ). Accordingly, 
At should be small enough to avoid aliasing. Practically it is advisable to take as 
large as = >(1.2 to 1.5)w., where w, is the uppermost frequency of the interested 
frequency component of the spectrum. Then 
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