What Do the Filter Structure Options in the Zero Phase Filter Mean in LabVIEW?

Updated May 3, 2018

Reported In

Software

  • LabVIEW
  • LabVIEW Digital Filter Design Toolkit

Issue Details

I saw in the Zero Phase Filter that the 'filter structure' options include IIR 2nd Order and IIR 4th Order. What do these options mean, and what's the difference between them?

Solution

'IIR' stands for Infinite Impulse Response. The 'order' of the IIR filter structure corresponds to the roll-off rate of the filter, which is defined to be 10X the rate of change of power (dB/decade) or 2X the rate of change of frequency (dB/octave) in the stop band. From this roll-off rate, we can then determine the width of the transition band (the frequencies between the passband and stopband). The roll-off rate is determined to be 20*n dB/decade, or 6n dB/octave for 'n' order filter, so for our options with the zero phase filter we have: 
  • IIR 2nd Order: 40 dB/decade or 12 dB/octave 
  • IIR 4th Order: 80 dB/decade or 24 dB/octave 

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