Quad-Band High-Temperature Superconducting Bandpass Filter Using Quadruple-Mode Square Ring Loaded Resonator

In this paper, a compact quad-band high-temperature superconducting (HTS) bandpass filter using a quadruple-mode square ring loaded resonator (SRLR) is introduced. The even- and odd-mode method is applied to investigate the equivalent circuits of the proposed quadruple-mode SRLR. The design graphs for the relationship of the parameters of electrical length and resonance performances are then set up. Based on the analysis above, four allocated resonant modes can be simultaneously excited and easily tuned. Meanwhile, multi-transmission zeros are created due to the different propagation paths of the square ring structure. Signal-interference theory is also adopted to explain the generating mechanism of transmission zeros. Moreover, a meander coupled-line technique is realized to adjust one uncertain resonant frequency to meet the target specifications of the designed quad-band filter. To verify this methodology, a second-order microstrip HTS filter operating at 2.45/3.5/5.2/5.8 GHz for wireless local area networks and worldwide interoperability for microwave access potential applications is designed using two quadruple-mode SRLRs. The quadruple-mode SRLRs are coupled with a pseudo-interdigital coupling structure for achieving the desired coupling degree conveniently, which also miniaturize the circuit size. This filter was fabricated on a 2-in-diameter 0.5-mm-thick MgO wafer with double-sided YBa2Cu3Oy thin films. The filter component was measured at the temperature of 77 K. Measured results agree with the theoretical results and show that the insertion losses in passbands are less than 0.3 dB, which exhibit superiority in midband insertion loss.