Characteristics of Ultra-Wideband Bandpass YBCO Filter With Impendence Stub

A compact ultra-wideband (UWB) bandpass filter (BPF) is presented for applications to short-range and high-speed wireless communication. Superconducting YBa₂Cu₃O_y (YBCO) stepped impedance resonators and coupled-line sections as inverter circuits are designed to form the basic filter structure. In the filter design, connected high-low stepped impedance microstrip lines construct the resonators, and open-stub lines are utilized to add return-loss poles in the pass-band and create transmission zeros in the lower/upper stop-band region. Simulation results show that the passband from 3.0 GHz to 8.6 GHz has a 3-dB fractional bandwidth of 99 percent, computed insertion losses better than 0.03 dB, and return losses greater than 15 dB. Rejection levels in the upper/lower stop-bands are better than 20 dB. For fabrication, high-T_c superconducting (HTS) YBCO films were deposited on double-side-polished 0.5-mm-thick MgO (100) substrates by a radio-frequency sputtering system. The filter was made out of patterned double-sided deposited YBCO films integrated with a gold-coated housing. The realized HTS UWB BPF shows a wide passband within 2.9-8.3 GHz with a maximum insertion loss of 0.88 dB. The measured results show good HTS UWB BPF performance. Moreover, the temperature-dependent frequency responses and the insertion loss can be described by the modified two-fluid-model-based formulas, indicating that the frequency shift and the increase in insertion loss for HTS BPF are both dominated by the temperature dependence of the magnetic penetration depth.