Strip-Loaded Slotline Resonator for Compact Differential-Mode Bandpass Filters with Improved Upper Stopband Performance

In this letter, a strip-loaded slotline resonator (SLSR) is presented and applied to design a class of novel differential mode (DM) bandpass filter (BPF) with improved upper stopband, compact size and intrinsic common-mode (CM) suppression. This DM BPF is composed of three paralleled coupled SLSRs, and two differential tapping-coupled ports. As the loaded-strip length is stretched, the 1
^st
resonant frequency gradually falls down so as to make the SLSR compact in size, while the 2
^nd
one is tremendously raised towards the 3
^rd
one so as to widen the spacing between the 1
^st
and 2
^nd
resonant frequencies or achieve a wide upper stopband of the proposed DM BPF. To verify the proposed technique, a third-order DM BPF prototype has been designed and fabricated. Both the simulated and measured results have demonstrated good DM upper stopband and CM rejection in addition to compact size.