Ultra-wide stopband HTS LPF with closed-loop cross-coupled microstrip structure

Abstract

A compact high-temperature superconducting (HTS) low-pass filter (LPF) with an ultra-wide stopband and a sharp transition band is designed by integrating the fifth-order low-pass filter branch and closed loop cross-coupled (CLCC) microstrip structure. The fifth-order low-pass filter consists of two radial-stubs (RSs), one butterfly-shaped stub (BSs), and two T-shaped stubs (TSs), which can introduce five transmission zeros (TZs) with independently controllable frequency and position, so as to obtain good sideband suppression or upper stopband characteristics. The CCLC microstrip circuit is composed of cascaded high and low impedance microstrip lines, which can widen the width of the upper stopband and realize the ultra-wide stopband characteristic. The operating mechanism of the fifth-order LPF resonator and the CLCC microstrip structure are analyzed by theLCequivalent circuit models. The implemented LPF on a 0.5-mm thick MgO wafer with double-sided YBCO films demonstrates 1-dB cut-off frequency (fc) at 2 GHz and 0.12 GHz narrow transition bandwidth from −1dB to −40 dB. The measured results show a 30-dB harmonic suppression from 1.08fc to 11.7fc i.e., above 23 GHz, a steep roll-off rate of 326 dB/GHz, an in-band insertion loss (IL) <0.10 dB, and a miniaturized size of 0.24λgc × 0.08λgc (λgc is the guided wavelength at the cut-off frequency).