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Chapter 4 Theory of Operation
© National Instruments Corporation 4-11 SCXI-1141/1142/1143 User Manual
Figure 4-8. Unit Step Response of the SCXI-1141/1142/1143 Module
Setting the Cutoff Frequency
The cutoff frequencies of the filters in the SCXI-1141/1142/1143 module
are set internally by dividing a base frequency of 100 kHz by an integer.
You can determine the allowable cutoff frequencies for the
SCXI-1141/1142/1143 module as follows:
where n is an integer ≥ 4 and f
c
≥ 10 Hz. In other words, f
c
= {25, 20, 16.7,
14.3, 12.5, ..., 0.01} kHz.
If you are using NI software, the software automatically chooses a divisor,
n, that best matches the cutoff frequency you specify and returns the actual
cutoff frequency chosen.
The correct cutoff frequency depends on the application. If phase
nonlinearity, ringing, passband ripple, or aliasing is a concern in the
application, you may need to set the cutoff frequency several times higher
0246810
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Time (Seconds)
Bessel (1142)
Butterworth (1143)
Elliptic (1141)
Output Voltage (V)
f
c
100
n
---------
kHz=