Design of a Linear-Phase HTS Filter With Different Transmission Zeros Introduced by Cul-de-Sac and CQ Structures

A method based on the cul-de-sac topology is proposed to simultaneously introduce a pair of controllable complex transmission zeros (TZs) on both sides of the passband. This topology maintains relatively strong internal coupling coefficients, thereby improving the design tolerance and robustness, which is particularly suitable for nontunable planar circuits. Furthermore, by analyzing the complementary characteristics of the cul-de-sac and coupled quadruplet (CQ) structures to intrude TZs, a hybrid linear-phase filter design method is developed, in which the two structures are combined to introduce complex and real TZs. A folded open-loop resonator supporting both electric and magnetic coupling is designed to integrate cul-de-sac and CQ units in a compact 14th-order high-temperature superconducting (HTS) filter with a footprint of$25.27\times 9.53$mm. The measured results at 4 GHz show a 7.5% bandwidth, in-band insertion loss below 0.15 dB, return loss better than −15.7 dB, and group delay under 5 ns across 75% of the passband, in good agreement with simulations.