Three element resonator lowpass filter

Abstract The conventional lowpass prototype filter with the finite transmission zeros close‐to‐band edge often yields negative circuit elements that lead to a practical difficulty of physical realizations. This paper presents an alternative topology of the lowpass prototype filter named three element resonator lowpass filter, which is composed of series inductances and shunt three element resonators. For one‐section three element resonator lowpass filter, the closed‐form solutions of the non‐negative circuit elements are deduced based on its characteristic function recovered from a prescribed transmission zero. For multi‐section one, the associated numerical synthesis procedure using gradient‐based optimization is presented. In physical realization, a much more practical configuration of three element resonator is proven by the equivalent circuit transformation. A synthesis example of lowpass prototype filter with 12 pairs of transmission zeros illustrates the effectiveness of the numerical synthesis method. Also, a design example of five‐section three element resonator lowpass filter, which has a passband up to 1775 MHz, is implemented by the distributed elements based on its prototype circuit of the numerical synthesis. The measured results of the fabricated lowpass filter experimentally validate the prototype filter.