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Technical Information
Aliasing Effect
The VM-83 directly converts the vibration input signal from analog into digital
form and performs signal processing in the digital domain, using a DSP. This
principle is subject to the so-called aliasing effect. An anti-aliasing filter (ana-
log low-pass filter) serves to counter this effect. In the VM-83, a 5th-order filter
is used for this purpose. Note that the filter has the effect shown below.
In the above illustration, the shaded section represents the area where the aliasing
effect is noticeable. With fs/2 as the center, the two spectrum representations
look like mirror reflections of each other.
The arrow (solid line) indicates a case where a signal 1.56 times the 20 kHz
upper measurement limit is input (31.2 kHz). In this case, aliasing will cause a
spectrum level detection of -50 dB (0.3%) at 20 kHz.
0 d B
- 5 0 d B
f c f s / 2
1 . 5 6 f c
f s
f
M e a s u r e m e n t r a n g e
f s : S a m p l i n g f r e q u e n c y 5 1 . 2 k H z
f c : H i g h e s t e f f e c t i v e f r e q u e n c y 2 0 k H z
f s = 2 . 5 6 f c
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