National Instruments 1142 用户手册

than the signal frequency range of interest. At frequencies much lower than 
the cutoff frequency, passband ripple and phase nonlinearity are much less 
noticeable. If you use the filter to prevent aliasing, you must set the cutoff 
frequency no higher than one-third of the frequency at which that channel 
is being sampled for the SCXI-1141 module, one-twelfth of the frequency 
for the SCXI-1142 module, or one-sixth of the frequency for the 
SCXI-1143 module.

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 4-9 shows an example of aliasing.

Figure 4-9.  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.

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Sampled Point

Reconstructed Signal

Input Signal