Miniature microstrip parallel-coupled bandpass filters based on lumped-distributed coupled-line sections

Novel miniature parallel-coupled bandpass filters are proposed, by using additional lumped capacitors to reduce the lengths of coupled-line sections. With the lumped-element K-inverters at the input/output and between coupled-line sections, the proposed filter may be equivalent to the quarter-wavelength resonator filter such that no repeated passband at twice the center frequency is observed. In addition, by introducing the capacitive cross-coupling effect, two transmission zeros can be achieved. The locations of the transmission zeros may be simply adjusted by varying the cross-coupling capacitance. Simple equivalent-circuit models along with design formulas are also established, from which the proposed parallel-coupled bandpass filters based on lumped-distributed coupled-line sections may be designed and investigated. Specifically, a miniature 4th-order microstrip bandpass filter with good selectivity and stopband rejection is implemented and carefully examined.