Design of Multifunctional Balanced-to-Single-Ended HTS Ultrawideband Filtering Balun for Suppressing Radio Frequency Interference

This letter introduces a high-temperature superconducting (HTS) ultrawideband balanced-to-single-ended filtering balun with the function of real-to-complex impedance transformation. This filtering balun is derived from a four-port symmetrical balanced common-mode filter. The schematic circuit is realized using HTS quasi-lumped elements. The hybrid whale gray wolf optimization algorithm is utilized to determine the element values in the equivalent circuit model, followed by optimizing the design of the layout of the filtering balun. To validate the design, a prototype filtering balun is fabricated with a differential-mode 1-dB passband ranging from 4.02 to 8.02 GHz and an impedance transformation from 50 to 100 +
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40 Ω. The common-mode rejection surpasses 35 dB within the passband.