Rion VM-83, Инструкция по эксплуатации, Страница: 87 / 106

77
Technical Information
Aliasing effect
The VM-83 directly converts the vibration input signal from analog into dig-
ital 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 fi lter
(analog low-pass fi lter) serves to counter this effect. In the VM-83, a 5th-order
fi lter is used for this purpose. Note that the fi lter 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 repre-
sentations look like mirror refl ections 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.