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APPLICATIONS INFORMATION
Aliasing
Any complex waveform, such as Slam Stick vibration
traces, can be expressed as the sum of sinusoidal
frequency components.
Per Shannon’s Sampling theorem, in any sampled data
such as that produced by the Slam Stick, the maximum
frequency that can be reliably identified is limited to half
the sampling frequency. This limiting frequency is
known as the Nyquist frequency, critical frequency or
folding frequency. Any frequency content at or above
Sampling devices such as the Slam Stick include a
lowpass filter at the folding frequency to minimize the
appearance of such aliases (see the Frequency
Response Graph in performance plot section);
however, no filter can completely eliminate unwanted
signals, so a very strong signal above the folding
frequency may still produce a visible alias. The best
course of action is to recognize that this can occur and
take steps to identify aliasing if it occurs. For example,
if the frequency content of the system is known to be
(or found to be) dependent on an external parameter
Figure 1: Aliasing occurs when the real signal frequency exceeds the Nyquist frequency,
resulting in there being not enough data points to correctly reconstruct the signal.
this limiting frequency will produce an alias, or false
signal appearing at a lower frequency. This alias will
appear as the reflection of the high frequency signal
over the folding frequency. For example, if the folding
frequency is 1600Hz (half of the Slam Stick’s 3200 Hz
sampling frequency), an 1800Hz signal would appear
as an alias at 1400Hz, and a 1900Hz signal would
appear at 1200Hz, etc. Figure 1 shows an example of
a real signal exceeding the folding frequency, and the
apparent (aliased) signal that results.
such as engine RPM, observing the response as this
parameter is varied can help in identifying spurious
signals.
Commercial Aircraft Vibration
Characterization
The following shows a brief example of using the Slam
Stick data to identify specific features in a recording.
This example uses the included MATLAB script for data
visualization.
REVISION N0. 001
REVISION DATE: 08-24-2010
www.mide.com
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