Design of a HTS low-pass filter with wide stopband suppression and in-band ultra-narrowband interference rejection

A low-pass filter capable of suppressing in-band ultra-narrowband interference (LPF-SUNI) is analyzed, designed, and implemented based on high-temperature superconducting (HTS) technology. By integrating a low-pass filter and a narrowband band-stop filter, the LPF-SUNI exhibits both ultra-narrowband stopband and wide stopband low-pass frequency response characteristics, enabling efficient and flexible suppression of interference signals either within the passband or in the adjacent out-of-band regions. First, using odd- and even-mode equivalent LC circuit analysis and transfer function synthesis, a folded interdigital coupled resonator is developed. Its transmission zero generation mechanism is derived to realize and verify a wide stopband low-pass filter with steep skirt characteristics. Next, by effectively employing the reactance/admittance slope parameter method, a half-wavelength stepped impedance resonator is designed with the introduction of series TZs, enabling the realization and measurement validation of a multi-zero ultra-narrowband band-stop filter. Finally, the LPF-SUNI is implemented on a YBCO/MgO/YBCO thin-film substrate. At 77 K, the LPF-SUNI measured -3 dB cutoff frequency is 1.53 GHz, out-of-band suppression greater than 60 dB, insertion loss (IL) below 0.2 dB, and return loss (RL) better than -10 dB. Moreover, an in-band interference signals centered at 1.42 GHz with an only 20 MHz bandwidth (1.41%) is effectively suppressed by 60 dB. The measured results show agreement with full-wave electromagnetic simulations.