The Poles of the Bessel filter can be found by solving a polynomial in s. The maths used here can be found in C. Britton Rorabaugh - Digital Filter Designer's Handbook - Publ by Mc Graw Hill.

There are 2 possible approximations. The first applet shows the case where the poles are chosen to produce an attenuation of 3dB at w = 1. The second applet shows the case where the poles are chosen for normalised group delay.

Magnitude is shown by red and pink lines, phase shift by green line and group delay by cyan line.

COPYRIGHT © 2005 Cuthbert Nyack.