Design of a Cul-de-sac Topology HTS Filter with Paired Transmission Zeros

By analyzing the mechanism of the Cul-de-sac topology to realize the response of the generalized Chebyshev response function, the principle of introducing the transmission zeros(TZs) of the transfer function is analyzed and discussed. Typical topological structure models and coupling matrices with different numbers of zeros and poles are extracted using the coupling matrix analysis method. By analyzing the tolerance and the magnitude characteristics of the coupling coefficient values of the filter's coupling coefficient, the suitability of the Cul-de-sac topology in high-temperature superconducting(HTS) filter applications is explained and elaborated. A Cul-de-sac topology HTS bandpass filter with a center frequency of 2345 MHz, a bandwidth of 50 MHz, and a pair of TZs was designed and fabricated on a 2-in-diameter 0.5-mm-thick MgO wafer with double-sided YBCO thin films. The machined physical circuit is then measured at 77 K, and the frequency response of the test results can be in good agreement with both the model applying the same principle and the frequency response of the full-wave electromagnetic simulation of the planar circuit. Therefore, the Cul-de-sac topology shows demonstrates the ease of implementation of coupling coefficients and good machining tolerance for the design of HTS filters.