Compact Lowpass Filter Design Using Cavity Resonator and Ladder-shaped Defected Ground Structure

A compact lowpass filter (LPF) design using cavity resonator and ladder-shaped defected ground structure (DGS) is presented in this paper. A cavity resonator is implemented on the top conductor layer with orthogonally oriented input and output feedlines for the proposed LPF filter. The cavity resonator has identical electrical function as of a square resonator. By proper design of the cavity structure, coupling coefficients and electrostatics parameters which influence the cutoff frequency can be controlled. The ladder-shaped DGS is etched on the ground plane directly beneath the 50 Omega input and output feedlines in order to manipulate the filter response. The DGS structure is effective for accurate transmission cutoff values, which ensure proper multiple spurious suppressions. The proposed LPF is prototyped using FR4 substrate with a dielectric constant of 4.7, loss tangent of 0.027 and thickness of 0.8 mm. The fabricated LPF has a compact dimensional size of 26 mm by 26 mm. The measured attenuation rate is 10 dB/GHz. Both simulation and measurement data are presented and discussed for the proposed LPF with and without the unique ladder-shaped DGS.