High Selectivity Wideband Superconducting Filter Design Based on Feed Line Resonator (FLR) Method

This letter presents the new concept and method of feed line resonator (FLR). Unlike the conventional filter feed lines, which serve solely for external coupling, the proposed feed line resonators can not only introduce additional transmission poles within the passband, but also generate transmission zeros at both sides of the passband, thereby significantly augmenting the filter's performance without enlarging its physical dimensions. A novel and systematic method for constructing multimode resonators, called the connected line resonator (CLR) method has been proposed, two parallel lines are connected to obtain three resonant modes for filter design. The key advantage of this proposed CLR is its ability to exhibit a substantial attenuation slope at the higher band edge. To demonstrate the FLR and CLR method, two wideband filters were designed based on the high-temperature superconducting film, with the center frequency at 7.3 GHz, and the fractional bandwidth of about 59%. The measured results agree well with the simulation ones.