Compact Differential Bandpass Filters Based on Coupled-Line Resonators with Improved Performance and Miniaturized Size

Two compact differential bandpass filters (BPFs) based on coupled-line resonators and loaded capacitors are proposed in this work. By properly designing the coupled resonator and the loaded capacitance of original filter model, differential-mode (DM) passband responses and commonmode (CM) rejection can be obtained. For validation, two differential BPFs named Filter I and Filter II are discussed and experimentally characterized. One more DM transmission zero generated by outof-phase cross-coupling is employed to control the DM bandwidth and sharpen the selectivity in Filter I while lumped capacitors are loaded in Filter II to replace capacitive coupled resonators for miniaturized size. Both filters are centered at 4.5GHz with about 9% DM fractional bandwidth (FBW), less than 1.5 dB insertion loss, more than 15 dB return loss, and wideband CM suppression with more than 18 dB rejection. Furthermore, the size of Filter II is substantially smaller than Filter I and previously reported differential BPFs.