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Butterworth filters

A filter with one real pole and one real zero can be configured as a shelving filter, as a high-pass filter (putting the zero at the point ) or as a low-pass filter (putting the zero at ). The frequency responses of these filters are quite blunt; in other words, the transition regions are wide. It is often desirable to get a sharper filter, either shelving, low- or high-pass, whose two bands are flatter and separated by a narrower transition region.

A procedure borrowed from the analog filtering world transforms real, one-pole, one-zero filters to corresponding Butterworth filters, which have narrower transition regions. This procedure is described clearly and elegantly in the last chapter of []. Since it involves passing from the discrete-time to the continuous-time domain, the derivation uses calculus; it also requires using notions of complex exponentiation and roots of unity which we are avoiding here.

To make a Butterworth filter out of a high-pass, low-pass, or shelving filter, suppose that either the pole or the zero is given by the expression

where is a parameter ranging from 1 to . If this is the point , and if it's .

Then, for reasons which will remain mysterious, we replace the point (whether pole or zero) by points given by:

where ranges over the values:

In other words, takes on equally spaced angles between and . The points are arranged in the complex plane as shown in Figure 8.17. They lie on a circle through the original real-valued point, which cuts the unit circle at right angles.

A good choice for a nominal cutoff or shelving frequency defined by these circular collections of poles or zeros is simply the spot where the circle intersects the unit circle, corresponding to . This gives the point

which, after some algebra, gives an angular frequency equal to

Figure 8.18, part (a), shows a pole-zero diagram and frequency response for a Butterworth low-pass filter with three poles and three zeros. Part (b) shows the frequency response of the low-pass filter and three other filters obtained by choosing different values of (and hence ) for the zeros, while leaving the poles stationary. As the zeros progress from to , the filter, which starts as a low-pass filter, becomes a shelving filter and then a high-pass one.

Next: Stretching the unit circle Up: Designing filters Previous: Peaking and band-stop filter   Contents   Index
Miller Puckette 2005-04-01