National Instruments FD-11634, Руководство пользователя, Страница: 23 / 55

Figure 15. Example Input Delay of a Lowpass Filter
Input Dela
y (s)
Frequency (Hz)
Input Delay
Maximum
Delay
Variation
Each FieldDAQ filter has a different frequency response to serve different applications:
•
Brickwall —Has a passband that extends to approximately 0.4 times the sampled data
rate, then drops with a very sharp transition band to reach the stopband and properly
reject any alias components or noise that may exist. The brickwall filter is designed to
provide full alias protection from any signal component of more than one-half the
sampled data rate (Nyquist frequency).
• Butterworth —Has a passband independent of the sampling rate (as opposed to the
brickwall and comb filters), which offers more flexibility when filtering out noise that is
below one-half of the sample rate. However, depending on your settings, you may see
alias components of higher frequency signals in your measurement that extend beyond
one-half of the sample rate due to the larger transition band.
• Comb —Has a smaller passband because its transition band starts early in the frequency
range. The comb filter has shorter group delay than other filters and better representation
of signals in the time domain (step response). The passband and transition band notches
track the data sample rate similar to the brickwall filter. The comb filter's transition band
features equally-spaced notches at different frequencies. It is common to use the comb
filter with a specific sample rate to align the notches of the transition band thereby
removing a specific noise-source frequency from measurements.
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