Compact dual-notched balanced UWB BPF with folded triple-mode slotline resonator

A compact dual-notched balanced ultra-wideband (UWB) bandpass filter (BPF) is designed by using the folded triple-mode slotline resonators and differential microstrip-slotline transitions. Two differential-mode transmission zeros near the passband edges are created to improve the selectivity, due to a cross-coupling between two microstrip open stubs beyond the cross-junction position as well as between the input and output. Good matching in the whole passband can be achieved by properly choosing the width of the microstrip open stubs. The inverse QWRs coupling stepped-impedance microstrip stub loaded in the 50 Ω microstrip feedlines can generate multiple transmission zeros to suppress the common-mode resonance and realise dual-notched bands in the UWB passband. Good agreement can be obtained between the measured results and simulated ones.