Eighteen-pole superconducting CQ filter for future wireless applications

A recent development of an eighteen-pole high-temperature-superconducting (HTS) bandpass filter with group delay self-equalisation for future wireless applications is reported. The filter is designed to have a 15 MHz pass-band at a centre frequency of 1967.5 MHz. The filter exhibits a quasi-elliptic function response implemented with a cascaded quadruplet (CQ) coupling structure. Two particular cross-coupled quadruplet realisations have been developed to implement either attenuation poles at finite frequencies or linear phase for group delay self-equalisation. The Monte-Carlo sensitivity analysis was performed and the results were used as guidelines for tuning the filter. The filter was fabricated on a 0.43 mm-thick sapphire wafer with double-sided YBCO films. Measured responses are presented to demonstrate the validity of the design as well as the potential of the filter.