Narrowband high temperature superconducting filter for mobile communication systems

The paper reports a recent development of a narrowband high temperature superconducting (FITS) bandpass filter for future mobile communication systems. The filter is designed to have a 5 MHz passband in the UNITS base station's receive band. The filter exhibits an 8-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 passband. It also exhibited an excellent rejection over the entire UMTS base station transmission band.