Signal aliasing

Equation (2.3.13) states the Nyquist sampling theorem. If At is set too high, the spectral width is too low, and signals at higher frequencies will appear at false positions (Fig. 2.3.9). This phenomenon is called signal aliasing [Deri]. 

To avoid signal aliasing, ky has to be stepped in small enough intervals so that the signal-phase increment Afcj,ymax for the maximum object coordinate y,nax never exceeds 2n. Similar considerations apply for the other space dimensions in x- and -directions. 

Signal aliasing due to excessive control interval (see Appendix B)... 

In addition, vibration data collected with a microprocessor-based analyzer is filtered and conditioned to eliminate non-recurring events and their associated vibration profiles. Anti-aliasing filters are incorporated into the analyzers specifically to remove spurious signals such as impacts. While the intent behind the use of anti-aliasing filters is valid, however, their use can distort a machine s vibration profile. 

Fig. 40.11. Aliasing or folding, (a) Sine of 8 Hz sampled at 16 Hz (Nyquist frequency), (b) Sine of 11 Hz sampled at 16 Hz (under-sampled), (c) A sine of 5 Hz fitted through the data points of signal (b).

Aliased signals Signals that fall outside the spectral window (i.e., those that fail to meet the Nyquist condition). Such signals still appear in the spectrum but at the wrong frequency because they become folded back into the spectrum and are characterised by being out of phase with respect to the other signals. 

Fig. 6.7. Block diagram of the differential mixed-signal architecture. AAF Anti-aliasing filter BF Buffer MUX Multiplexer...

Fig. 5. Multiple aliasing for 2Q-HoMQC of a 24-mer RNA hairpin 7], The spectrum was acquired in D2O with presaturation of the residual water signal. Relevant sections of the 2Q pseudo-diagonal are denoted with solid line (no aliasing), dashed line (single aliasing),...

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