Narrow-band HTS filter on sapphire substrate

This paper presents a narrow-band HTS bandpass filter on sapphire substrate for future mobile communications systems. The filter is designed to have a five MHz pass band in the UMTS base stations receive band. The filter exhibits a eight-pole quasi-elliptic function response implemented with a cascaded quadruplet coupling structure. The filter was fabricated on a 0.43 mm-thick sapphire wafer with double-sided YBCO films. The measured filter showed a midband insertion loss of 0.5 dB at 1973 MHz and a return loss better than -15 dB over the pass band. It also exhibited an excellent rejection over the entire UMTS base station transmission band.