Compact UWB bandpass filter on hybrid microstrip/slotline structure with improved out-of-band performances

A compact ultra-wideband (UWB) bandpass filter with good in-band and out-of-band performances is proposed and implemented on hybrid microstrip/slotline structure. A multiple-mode slotline resonator with stepped-impedance configuration is first properly constructed, aiming to allocate its first three resonances occurring at desired positions within the UWB passband. Two 50 Omega microstrip feed lines are then placed perpendicular to this slotline resonator on the opposite side of the substrate. As a result, an initial back-to-back microstrip-slotline bandpass filter is constituted with the targeted UWB passband. Later the slotline resonator is twisted and an additional coupling between two open-ends of feed lines is introduced, resulting in miniaturizing its overall circuitry size and enhancing the roll-off skirts out of the passband. In final, defected ground structures (DGSs) are formed on the two feed lines to improve the upper stopband performance. Two UWB bandpass filters are designed, fabricated and measured. Measured results demonstrate an UWB passband from 3.25 to 10.21 GHz with in-band return loss >10.43 dB and group delay variation <0.34 ns.