A compact elliptic-function low-pass filter using stepped-impedance coupled microstrip resonators

A novel elliptic-function low-pass filter (LPF) with a compact size and a sharp roll-off skirt characteristic using stepped-impedance coupled microstrip resonators (SICRs) is proposed. The proposed LPF is investigated based on a proper ideal lumped equivalent-circuit model and the sharp skirt selectivity is obtained through multiple transmission zeros generation using SICRs. A compact elliptic-function eleventh order LPF of dimensions 0.23λ_g × 0.31λ_g (λ_g is the guide wavelength at cutoff frequency) was fabricated and tested for performance confirmation. The fabricated LPF with 1-dB cutoff frequency of 2.5 GHz demonstrates an excellent out-of-band performance with more than 40-dB rejection from 2.9 to 6.5 GHz. The measured results demonstrate good agreement between design, simulation, and measurement.