High-Order HTS Dual-Band Differential Bandpass Filter Using Stub-Loaded Twin-Ring Resonator With Fully Controllable Passbands

In this paper, a newly stub-loaded twin-ring resonator (SLTRR) with symmetrical structure and multimode property is proposed and based on it, a sixth-order high-temperature superconducting (HTS) dual-band differential bandpass filter (BPF) with full controllable passbands is designed. The transmission line model (TLM) of the symmetrical SLTRR is constructed and its resonant characteristics are well analyzed with the help of odd/even-mode method. By proper loading six open-circuited stubs on the twin-ring, two individually adjustable DM frequencies can be obtained and utilized to constitute two DM passbands. Moreover, two open-circuited stubs installed at upper and lower portion of twin-ring are replaced by the shunted-lines, leading to the abundant design freedom for controlling two coupling coefficients of dual DM passbands separately. Furthermore, the required two external quality factors can be obtained independently by the proposed dual-band feeding structure. Thus, the center frequencies and bandwidths of two DM passbands can be fully controllable. Besides, a improved CM suppression within the second DM passband is realized by using frequency discrepancy technique. Finally, a dual-band differential BPF with center frequencies of 2.50/5.20 GHz is designed and fabricated on MgO with double deposited YBCO films. The measured results agree well with the simulated ones, and the filter has the advantages of high selectivity, low insertion loss, design flexibility and favorable CM suppression.