Kelly K. Johnson Irwin W. Sandberg The stability of quadratic filters. 191-197 1998 26 Int. J. Circuit Theory Appl. 2 https://doi.org/10.1002/(SICI)1097-007X(199803/04)26:2<191::AID-CTA996>3.0.CO;2-X db/journals/ijcta/ijcta26.html#JohnsonS98