New quasi-lumped resonators for compact microstrip filter design

In this work, a new type of quasi-lumped resonators is proposed for the design of microstrip filters with compact dimensions. These resonators consist on two metallic connections between the conductor strip and the back metal side (through the use of wires and vias) and a ring etched in the ground plane, surrounding one of such connections. The etched ring in the ground plane introduces a grounded (edge) capacitance to the structure which is connected to the upper conductor strip through the inductance provided by the via-wire combination. Since the other metallic connection emulates an additional inductance to ground, the structure is properly described by the parallel combination of a shunt inductor and a series LC resonator. By coupling these quasi-lumped resonant structures with quarter wavelength transmission lines (acting as admittance inverters), it is possible to synthesize a wide variety of planar filters with controllable bandwidth and compact dimensions. Several prototype device examples are provided to illustrate the potentiality of the approach for the design of microstrip band pass filters.