Dual-Band Differential Bandpass Filters Using Quadruple-Mode Stubs-Loaded Ring Resonator With Intrinsic Common-Mode Suppression for 5G

A novel quadruple-mode stubs-loaded ring resonator (SLRR) is presented in this letter, which can achieve compact dual-band differential bandpass filters (BPFs) with intrinsic common-mode suppression. The differential-mode (DM) equivalent circuit and common-mode (CM) equivalent circuit of the SLRR are given and investigated by using the even-/odd-mode theory. Two DM resonances of the SLRR can be adjusted independently and are used to construct two DM passbands. Due to the magnetic-dominated coupling between two SLRRs, the intrinsic CM suppression characteristics are achieved in the differential passbands without using any additionally loaded elements. And by loading the different short-circuited coupled lines (SCCLs) at the parallel-coupled feed-line of differential BPF separately, two transmission zeros (TZs) are introduced on both sides of the DM passbands, which improves the selectivity of dual differential passbands. Furthermore, to realize the desired external Q values for two DM passbands, hybrid feeding line structure including tapped and coupled-line feeding structures is used to produce the valid excitation for the differential BPF. At the same time, the open stubs are attached to the inside of the ring resonator to reduce the filter size. Finally, two dual-band differential BPFs with central frequencies of 3.5 GHz and 4.9 GHz are designed and fabricated for fifth generation of mobile communications (5G). The measured minimum CM suppression of the two DM passbands are 31.0 dB and 34.8 dB.