Quasi-Elliptic Function HTS Filter With Controllable Transmission Zeros

A miniaturized eight-order quasi-elliptic function filter configuration with controllable transmission zeros (TZs) is proposed. Without changing the overall layout of the filter, two pairs of TZs are introduced by adopting two cascaded quadruplet (CQ) units to improve the suppression of the filter. At the same time, a cross capacitance coupling is introduced to combine the two CQ units to realize the synchronous adjustment of these two pairs of TZs. Through using this configuration, it not only helps realize the miniaturization of the filter, but also reduces the complexity of adjusting the two CQ units individually. Finally, an eight-order high-temperature superconducting filter with two pairs of TZs is designed on a double-sided YBCO/MgO/YBCO thin film with a size of 15.95 mm × 7.91 mm, a thickness of 0.5 mm, and a dielectric constant of 9.8. At 77 K, the measured central frequency of the filter is 2000 MHz, the bandwidth is 35 MHz, the out-of-band rejection is greater than 60 dB/MHz, the insertion loss is less than 0.22 dB, the return loss is better than 15.8 dB, and the test results are in good agreement with the simulated ones.