Novel semi-lumped circuit model of 4 poles dual-mode quasi-elliptical resonator filter for Wimax applications

A novel semi-lumped circuit model of 4 poles dual-mode quasi-elliptical resonator filter was proposed and demonstrated in Wimax filter design. Two orders of semi-lumped closed loop resonator were electromagnetically coupled through coupled-line section and four shunt capacitors as perturbation elements for dual mode resonator excitation. With tuning the ratio of even/odd mode impedance of coupled line, the bandwidth and selectivity are adjustable, and two transmission zeros aside centre band are also controlled by perturbation capacitors C_p1 and C_p2 as well. The filter design was commercially fabricated with FR-4 PCB assembly associated with commercial SMD MLCC capacitors and MLCI inductors and coupled-line trace designed as broadside coupled strip-line. The filter performance yields good selectivity over 40 dB at stop-band and narrow FBW near 4%. With taking several exact passive modeling S-parameters database into account in the filter design, there´s only a significant discrepancy on measurement due to SMD component´s parasitic contribution [6], [7] which do lowering the filter circuit Q, results in poor insertion loss and return loss as well.