BSA Flow Software Installation and User's Guide - CSI

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If the true signal contain components with frequencies higher than the
Nyquist critical frequency, they will be falsely interpreted as lower
frequency components with the same signal intensity.
(7-73)
To avoid this so-called aliasing the Nyquist criterion specify that samples
should be taken at least twice in each period of the highest frequency
contained in the sampled signal.
In the context of spectrum calculation aliasing mean that frequencies higher
than half the sampling frequency will falsely contribute to the spectrum at
lower frequencies.
Consider for example the signal shown in Figure 7-86: The true signal
consists of three components: A 60Hz component, a 150Hz component and
some white noise.
Low frequency component
High frequency component
True signal with noise band
T2=1/f2
T1=1/f1
Figure 7-86 : The high frequency component drown in noise,
leaving only the low frequency component visible.
On an oscilloscope the 60Hz component will be clearly visible, but the
150Hz component drown in noise and may not be recognized. If the
sampling frequency is chosen as 240Hz, the Nyquist criterion state, that
frequencies up to 120Hz will be resolved. This is more than enough to
resolve the 60Hz component, but the energy contained in the 150Hz
component will be aliased, and contribute to the spectrum around 90Hz as
shown in Figure 7-87:
BSA Flow Software:Reference guide 7-129