Novel compact microstrip parallel-coupled bandpass filters with lumped-element J-inverter

Novel compact parallel-coupled bandpass filters are proposed, by using additional lumped-element capacitor as J-inverter between short-ended coupled-line sections to achieve an equivalent to the quarter-wavelength resonator filters. As a result, the filter order can be doubled without increasing the circuit area, and no repeated passband at twice the center frequency is observed. In addition, lower power loss is obtained compared to the previous works based on open-ended coupled-line sections. By introducing an inductive cross-coupling effect, two transmission zeros at upper and lower stopbands may also be created. The locations of the transmission zeros may be simply adjusted by varying the cross-coupling inductance. Simple equivalent-circuit models along with design equations are also established, from which the proposed parallel-coupled bandpass filters with lumped-element J-inverter may be designed and investigated. Specifically, several compact 2nd-order microstrip bandpass filters with good selectivity are implemented and carefully examined.