©
National Instruments Corporation
15
SCC-LP Series Lowpass Filter Modules User Guide
Using the SCC-LP
XX
as an Antialiasing Filter
Aliasing, a phenomenon of voltage-sampling systems, causes a
high-frequency signal to take on the identity of a low-frequency signal.
Figure 7.
Aliasing of an Input Signal Frequency of 0.8 Times the Sampling Rate
The solid line depicts a high-frequency signal being sampled at the
indicated points. However, when these points are connected to reconstruct
the waveform, as shown by the dotted line, the signal appears to have a
lower frequency. Any signal with a frequency greater than one half of the
sampling rate will be aliased and incorrectly analyzed as having a
frequency below one half of the sampling rate. This limiting frequency,
one half of the sampling rate, is known as the
Nyquist frequency
.
To prevent aliasing, you must remove all of the signal components with
frequencies greater than the Nyquist frequency from an input signal
before
you sample it. When you sample the data and aliasing occurs, it is
impossible to accurately reconstruct the original signal.
The SCC-LP
XX
removes these high-frequency signals before they reach
the E Series DAQ device and cause aliasing. Because the SCC-LP
XX
stopband begins at ten times the cutoff frequency (for an attenuation of
80 dB), the Nyquist frequency should be at least ten times the cutoff
frequency. Thus, the rate at which the E Series DAQ device samples a
channel should be at least 20 times the filter cutoff frequency.
For example, if you use the SCC-LP01, which has a cutoff frequency of
25 Hz, you can calculate the
minimum
scan rate used by the E Series DAQ
device to prevent aliasing
—
25 Hz
×
20 = 500 samples per second per
channel.
0
2
4
6
8
10
1
–1
Sampled Points
Reconstructed Signal
Input Signal
Summary of Contents for SCC-PWR02
Page 1: ...SCC PWR02...
