National Instruments 1141, 1142, 1143, SCXI - page 38

Chapter 4 Theory of Operation
© National Instruments Corporation 4-7 SCXI-1141/1142/1143 User Manual
As Figure 4-4 shows, the SCXI-1141/1142/1143 module provides 80 dB
attenuation above 1.5 times the cutoff frequency for the SCXI-1141
module, six times for the SCXI-1142 module, and 3.2 times for the
SCXI-1143 module. The SCXI-1141, which incorporates an elliptic filter,
is designed to provide maximum attenuation immediately above the cutoff
frequency. Therefore, it is the ideal choice for applications in which you
must remove signals very near the cutoff frequency.
Figure 4-4. Typical Magnitude Response of the SCXI-1141/1142/1143 Module Filters
Figure4-5 compares the magnitude response of the SCXI-1141/1142/1143
modules within the passband. The passband magnitude response begins to
drop off immediately for the SCXI-1142 module. The SCXI-1141 performs
much better than the SCXI-1142 in the passband, but it still exhibits about
0.1 dB of ripple in magnitude in the passband. The SCXI-1143 module
Butterworth filter is designed for maximum flatness in the passband and is
nearly perfectly flat in most of the passband. For this reason the SCXI-1143
module filter is the ideal choice for applications where flatness in the
passband is critical.
1 x 10–3
Bessel (1142)
–80 dB Intersection
Elliptic (1141)
0.01 0.1 1 10
–200
–150
–100
–50
0
50
Frequency (Normalized)
Gain (dB)
–80
Butterworth (1143)