Chapter 4
Theory of Operation
©
National Instruments Corporation
4-13
SCXI-1141/1142/1143 User Manual
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
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.
f
c
100
n
--------- kHz
=
