National Instruments 1142 用户手册

You can set the cutoff frequencies of filters in the SCXI-1141/1142/1143 
module by using the external clock input in applications that require 
external control of the cutoff frequency or that require finer resolution than 
the module provides internally. The cutoff frequency for each filter using 
the external clock as a base is:

/(100

 × n)

where f

 is the frequency of the external clock and n is an integer you 

select such that 2 

≤ n ≤ 2

16

.

When the frequency of the external clock changes, the cutoff frequency 
changes proportionally.

An external clock can control the SCXI-1141/1142/1143 module filters 
because they use a switched-capacitor architecture, which uses analog 
sampling. However, this technique is also susceptible to aliasing in much 
the same way as the digital sampling of a DAQ device (with a Nyquist 
frequency of one-half the external clock frequency). Analog sampling also 
creates high-frequency images of the signal because the output waveform 
has a staircase shape.

The SCXI-1141/1142/1143 module prevents these errors by using sets of 
prefilters and postfilters that do not sample the signal. A different set of 
prefilters and postfilters is used for each of 12 ranges of input frequencies. 
The prefilters reduce signals that can alias into a lower frequency by at least 
40 dB, and the postfilters reconstruct the output waveform, reducing 
high-frequency images to at least –80 dB.

NI software automatically chooses the correct set of prefilters and 
postfilters when you specify a cutoff frequency. However, when the 
external clock input is used to set the cutoff frequency of a filter, you must 
still supply an approximate cutoff frequency so that the software can 
determine the appropriate set of prefilters and postfilters.

Table 4-1 gives the ranges of cutoff frequencies that the prefilters and 
postfilters use.