Development of compact cross-coupled filters for mobile communications

Investigations on compact devices developed for the second (2G) and the third (3G) mobile communications systems are presented in this paper. Four-pole and six-pole filters were designed by cross-coupling new compact planar resonators. The coupling constants were investigated as functions of coupling gaps. Several models of the pass-band filters were manufactured and characterized. They exhibit steeper responses due to the cross-couplings, which introduce transmission zeros on each side of the pass-band. Moreover, due to their shapes, the resonators exhibit a first high order response at more than twice the fundamental frequency, therefore the filters show an enlarged rejection band. The measured filter responses are in a good agreement with the simulated responses. The developed filters are compact, cost-effective and suitable to be designed on high dielectric constant ceramic substrates, or for high temperature superconducting (HTS) circuits.