Design and development of ferroelectric tunable HTS microstrip filters for Ku- and K-band applications

In the past few years, our group has designed, fabricated and tested several ferroelectric tunable components, in collaboration with NASA Glenn Research Center. Among the components studied are microstrip tunable resonators, filters, diplexers, and couplers. Tunable filters with gold as well as YBCO high temperature superconductor (HTS) electrodes have been studied using the conductor/ferroelectric/dielectric two-layered microstrip configuration. Tunability is achieved through the non-linear dc electric field dependence of the relative dielectric constant of the ferroelectric thin film. Two pole ferroelectric tunable HTS filters using the YBCO/STO/LAO two-layered microstrip yielded low insertion loss as low as 1.5 dB, and large frequency tunability (∼2.3 GHz) at 24 K, with a peak electric field of 80 kV cm−1 for electrical tuning. The critical design issues for realizing practical tunable filters will be addressed. These include design for (i) large tunability, (ii) low insertion loss, (iii) bandwidth considerations, and (iv) dc biasing for tuning the filters. HTS filters will be compared with their gold counter parts for their performance at Ku- and K-band frequencies. Also, our current work on BSTO based room temperature tunable filters will be presented.