Design Methods for the Classical Filter

The paper describes the central concept of a classical filter which is defined by the following features: a lossless 2-port between resistive terminations; individually tunable attenuation poles; as many attenuation zeroes as feasible. This set of three conditions has an unexpected side result: the unsensitivity of the filter to variations of its components up to the first order. This explains a posteriori why it was possible to build, during fifty years, filters out of coils and condensers which were relatively imprecise, subject to aging and temperature effects. The concept of the classical filter emerged when the designers tried to depart from this solution. The first attempts to build RC active filters were total failures till the designers learned to simulate the classical filter. Hence this concept has an interest in its own right and it transcends the border between lumped and distributed circuits.