Design of an Ultrawide Stopband HTS LPF Based on Interdigital Hairpin Structure

Abstract

A novel compact high-temperature superconducting low-pass filter (LPF) with an ultrawide stopband, excellent out-of-band rejection, and a low insertion loss is presented. This filter consists of a four-order LPF constructed by four interdigital hairpin stepped-impedance resonators (IHSIRs) and two pairs of radial stubs (RSs) in the input and output ports of the filter. The four-order IHSIR LPF brings transmission zeros for a high selectivity, and the RSs exhibit a characteristic of extending the LPF’s stopband effectively. The proposed LPF with cutoff frequency $f_{c}$ at 3 GHz is implemented on a 0.5-mm-thick MgO wafer with double-sided YBCO films. The measured results show a 30-dB harmonic suppression from $1.08f_{c}$ to $7f_{c}$ , a steep roll-off of 197.1 dB/GHz, and an in-band insertion loss less than 0.13 dB. Moreover, the LPF possesses a miniaturized size of $0.21\lambda _{\text {gc}} \times 0.03\lambda _{\text {gc}}$ .