Chapter 4
Theory of Operation
SCXI-1141/1142/1143 User Manual
4-14
ni.com
Using the SCXI-1141/1142/1143 Module as an Antialiasing Filter
Aliasing, a phenomenon of sampled data acquisition systems, causes a
high-frequency signal component to take on the identity of a low-frequency
signal. Figure 4-10 shows an example of aliasing.
Figure 4-10.
Aliasing of an Input Signal with a Frequency 0.8 Times the Sample 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 frequency with a frequency component greater
than one-half of the sample rate is aliased and incorrectly analyzed as
having a frequency below one-half of the sample rate. This limiting
frequency of one-half the sample rate is known as the
Nyquist frequency
.
To prevent aliasing, you must remove all signal components with
frequencies greater than the Nyquist frequency
before
sampling an input
signaled. After an unfiltered signal is sampled and aliasing has occurred, it
is impossible to accurately reconstruct the original signal. The
SCXI-1141/1142/1143 module removes these high-frequency signals
before they reach a DAQ device and cause aliasing.
Because the SCXI-1141 module stopband begins at 1.5 times the cutoff
frequency, the Nyquist frequency should be at least 1.5 times the cutoff
frequency. Thus, the rate at which the DAQ device samples a channel
should be at least three times the filter cutoff frequency to acquire
meaningful data.
The stopband for the SCXI-1142 module begins at six times the cutoff
frequency, so you should sample it at a rate of 12 times the cutoff frequency
to acquire meaningful data.
0
2
4
6
8
10
1
–1
sampled point
reconstructed signal
input signa
l
