Chapter 4
Theory of Operation
©
National Instruments Corporation
4-9
SCXI-1141/1142/1143 User Manual
Figure 4-5.
Typical Passband Responses of the SCXI-1141/1142/1143 Module
Phase Response
Figures 4-6 through 4-9 illustrate the phase response characteristics of the
SCXI-1141/1142/1143 module filters. Figure 4-6 shows the phase shift as
a function of frequency (normalized so that the cutoff frequency = 1). In an
ideal filter, this would be a linear relationship. Figure 4-7 shows the
deviation of the actual phase response from an ideal (linear) response.
Generally, phase response is described in terms of the differential
nonlinearity, or group delay. Group delay is defined as the negative
derivative of the phase shift with respect to the frequency. In the ideal filter,
group delay is a constant. The group delay of the SCXI-1141/1142/1143
module filters is shown in Figure 4-8.
0
0.2
0.4
0.6
0.8
1
–3.5
–3
–2.5
–2
–1.5
–1
–0.5
0
0.5
Bessel (1142)
Butterworth (1143)
Elliptic (1141)
Frequency (normalized)
Gain
(dB)
