Development of L-band and C-band superconducting planar filters for wireless systems

We report on the progress in the fabrication process of high temperature superconducting (HTS) films and the design and realisation of planar filters for both receiving and transmitting telecommunications systems. Double-sided thin films having dimensions ranging from 1 to 5 cm are grown fully in situ on (100) LaAlO₃ substrates by a high oxygen pressure d.c. sputtering technique. Deposition of highly epitaxial c-axis films of YBa₂Cu₃O_7-δ (YBCO), YNd_xBa_2-xCu₃O_7-δ (Y-NBCO) and Nd_1+xBa_2-xCu₃O_7-δ (NBCO) having critical temperatures of about 88 K and very low values of surface losses is routinely carried out. Multipole L-band forward coupled filters with a Tchebischev response having the whole 75 MHz band or a 25 MHz wide sub-band of the DCS 1800 frequency allocation have been designed and tested. Dual mode C-band filters based on cross slotted square resonators with a miniaturised size and high power handling capability are also presented. Device characteristics have been studied as a function of operating temperature and input power. The linearity in the filter response has been also tested by measuring two-tone intermodulation distortion