The formulation of criteria of BIBO stability of 3rd-order IIR digital filters in space of coefficients of a denominator of transfer function

Criterion for the BIBO stability of the IIR digital filters is well known. IIR filter is BIBO stable if and only if all of its poles are strictly inside the unit circle in the complex z-plane. To analyze the stability of biquad filters in the space of coefficients is constructed the famous “triangle of stability”. In this paper, this criterion extends on third order IIR digital filters.