IIR Direct method with MIP and Tan Low Pass.

Cuthbert Nyack
The frequency response and Z plane poles for a low pass IIR filter with mirror image poles generated by the tan is shown in green in the applet below.

To design a filter with a gain of -1dB at 0.125ws and -50dB at 0.25ws. First set BL = 94 and WL = 188 to locate the frequencies. Now adjust N and fc/fs until the requirements are met. This gives N = 8 and fc/fs = 0.133.
Changing nP shows the numerical values of the poles and changing Fn to 1 shows the quadratic factors necessary to realize the filter.

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COPYRIGHT 1999,2008 Cuthbert A. Nyack.