and there is a zero at and another at . If the
damping is sufficienly small so that
, then
the poles form a complex-conjugate pair:

Since
, the poles are always in the left-half
plane, and hence the analog RLC filter is always stable. When the
damping is zero, the poles go to the axis:

This book is a gentle introduction to digital filters, including mathematical theory, illustrative examples, some audio applications, and useful software starting points.