Stepped-Impedance Coupled Resonators for Implementation of Parallel Coupled Microstrip Filters With Spurious Band Suppression

A parallel-coupled microstrip bandpass filter based on the stepped-impedance coupled resonator (SICR) for suppression of harmonic spurious response is presented. The SICR relies upon the incorporation of different stepped impedances for the even and odd propagation modes of the coupled microstrip lines. This is to align the modal parallel resonance frequencies, and thereby effectively equalize the modal phase velocities. Its structural simplicity enables closed-form analytical formulation, and the derived equations are effectively utilized to explicitly guide parameters selection based on classical synthesis methodology of coupled microstrip filters. Practical feasibility of the SICR and the integrity of the analysis are demonstrated via designs and implementations of two third-order 0.01-dB ripple Chebyshev bandpass filters with a passband center frequency of 2.0 GHz and 3-dB fractional bandwidths of 15% and 25%. Good agreement between design, simulation, and measurement is obtained, where suppressions of the first harmonic spurious responses by more than 40 dB are achieved in both filters.