A YBCO/BST/MgO interdigital varactor and an L-band tunable HTS bandpass filter

This paper presented a YBCO/Ba_0.05Sr_0.95TiO₃ (BST-0.05) heterostructure thin film interdigital varactor grown on a MgO substrate by pulsed laser deposition (PLD). The microwave properties of the BST film were characterized at cryogenic temperature. The dielectric constant changed 78 % from 1160 to 252 and loss tangent was in the range of 0.01-0.02. An L-band four pole tunable high temperature superconductor (HTS) bandpass filter with semiconductor varactors as tuning elements was also presented in this paper. For a dc bias voltage of 0.5-8 V, the filter center frequency tuned from 1.25 GHz to 1.58 GHz, with an insertion loss of 1.7-1 dB, a 40 dB/3 dB shape factor better than 3.7, and a reflection better than -10.4 dB.