The information in this app note is not correct: http://www.analog.com/media/en/training-seminars/tutorials/MT-204.pdf

- Bessel filters have a small amount of overshoot and ringing, not zero. (Purely real pole filters and Gaussian filters have zero overshoot.)
- The poles are the roots of the Bessel polynomials, which are neither on a circle nor equally spaced along the imaginary axis. They are in an arc that looks
*somewhat*circular, but not exactly, and the center of the circle would be in the right-hand plane, not at 0:

The pole locations in Figure 7. Bessel Design Table are correct, but if you plot one of them you'll see they aren't on a circle, and the imaginary parts are not equally spaced:

2.2994-1.7395 = 0.5599

0.7335-0.2451 = 0.4884

.