Page 233 - Anatomy of a Robot

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218 CHAPTER EIGHT
Figure 8-20 is from a twelfth-order Butterworth IIR filter. It too drops about 20 db
in a frequency range of about 0.050 normalized.
But the IIR filter is just twelfth order, made out of a series of second-order IIR fil-
ters. A second-order filter can take many different structures. One example is
shown in Figure 8-21. Each order is the hardware equivalent of about 2 FIR taps,
so a twelfth-order IIR filter is the equivalent of about 24 FIR taps, shorter for the
same results.

FIGURE 8-20 DSP FIR filter frequency response with a twelfth-order filter

Coefficient b1
X

Tap Register Tap Coefficient b2
X

Tap Register Tap Coefficient b2
X

Tap Coefficient a1
X

+
Tap Coefficient a2
X Output

Input +

Feedback Path
FIGURE 8-21 A second-order IIR filter