#### Butterworth Lowpass Poles and Zeros

When the maximally flat optimality criterion is applied to the general (analog) squared amplitude response , a surprisingly simple result is obtained [64]:

 (I.1)

where is the desired order (number of poles). This simple result is obtained when the response is taken to be maximally flat at as well as dc (i.e., when both and are maximally flat at dc).I.1Also, an arbitrary scale factor for has been set such that the cut-off frequency (-3dB frequency) is rad/sec.

The analytic continuationD.2) of to the whole -plane may be obtained by substituting to obtain

The poles of this expression are simply the roots of unity when is odd, and the roots of when is even. Half of these poles are in the left-half -plane ( re) and thus belong to (which must be stable). The other half belong to . In summary, the poles of an th-order Butterworth lowpass prototype are located in the -plane at , where [64, p. 168]

 (I.2)

with

for . These poles may be quickly found graphically by placing poles uniformly distributed around the unit circle (in the plane, not the plane--this is not a frequency axis) in such a way that each complex pole has a complex-conjugate counterpart.

A Butterworth lowpass filter additionally has zeros at . Under the bilinear transform , these all map to the point , which determines the numerator of the digital filter as .

Given the poles and zeros of the analog prototype, it is straightforward to convert to digital form by means of the bilinear transformation.

Next Section:
Example: Second-Order Butterworth Lowpass
Previous Section:
Mechanical Equivalent of an Inductor is a Mass