We have developed a 2 GHz band superconducting 8-pole tunable quasi-elliptic function filter on sapphire substrate. The tunable filter has a sharp skirt characteristic by transmission zeros. And the tunable filter uses a low-cost substrate of sapphire. An adjustment of center frequency for the filter is realized by a tunable technique with a piezoelectric bending actuator. The tunable filter realized center frequency shift of 7.62 MHz with conditions of bandwidth change of 0.5% and ripple change of 0.25 dB. Center frequency of the 8-pole quasi-elliptic function filter agreed with the designed value. Given these features, a superconducting filter with a sharp skirt characteristic for next-generation mobile RF applications is expected to be realized by applying these filters.

We have developed a high-temperature superconducting (HTS) filter with narrow bandwidth characteristic for receiver of weather radar in order to reduce interference between adjacent radar channels. To realize a filter with which a narrow bandwidth and low insertion loss are compatible, resonators with high unloaded Q (Qu) value are required. Hairpin microstrip resonators with 1.5 times wavelength were adopted to suppress the radiation loss and achieve a high Qu value. The developed HTS filter has 8-pole quasi-elliptic function response for sharp skirt characteristic. The measured frequency response of the developed filter shows center frequency of 5370 MHz, insertion loss of 2.04 dB and maximum return loss of 15 dB, which agrees with the designed responses.

2 GHz band electrically tunable superconducting microstrip line band-pass filter was developed. The tunable filter used a thin interdigital electrode. The dielectric distribution of SrTiO3 substrate included a calculated nonlinear effect of the electrode and ferroelectric material. As a result, the tunable microstrip line filter design with interdigital electrode enabled calculation by the finite element method and the moment method. The tunable filter with a measured unloaded Q factor of 9700 and a frequency shift of 1.25 MHz was obtained.

We developed a compact rf receiver subsystem using a high-Tc superconducting sharp skirt band-pass filter with a center frequency tuning function. A 24-pole hairpin-type 2 GHz microstrip-line filter was fabricated with YBa2Cu3Oy thin films deposited on a LaAlO3 substrate. Attenuation characteristics were more than 30 dB at 1 MHz apart from both the lower and the higher pass-band edges. For center frequency tuning, a 1-mm-thick dielectric sapphire plate was stacked on the filter, and the filtering characteristics were tuned by moving the plate using a piezoelectric bending actuator. The range of the center frequency modulation was more than 12 MHz with no degradation of the low-loss and sharp-skirt characteristics.

A monolithic linear power amplifier IC operating with a single low 2.7-V supply has been developed for 1.9-GHz digital mobile communication systems, such as the Japanese personal handy phone system (PHS). Refractory WNx/W self-aligned gate GaAs power MESFETs have been successfully developed for L-band power amplification, and this power amplifier operates with high efficiency and low distortion at a low voltage of 2.7 V, without any additional negative voltage supply, by virtue of small drain knee voltage, high transconductance and sufficient breakdown voltage of the power MESFET. An output power of 23.0 dBm and a high power-added efficiency of 30.8% were attained for 1.9-GHz π/4-shifted QPSK (quadrature phase shift keying) modulated input when adjacent channel leakage power level was less than -60 dBc at 600 kHz apart from 1.9 GHz.