Four-Pole Narrowband Superconducting Tunable Filter at VHF-Band

This paper presents a narrowband superconducting tunable filter at very high frequency (VHF)-band with a fractional bandwidth of 1.4%. The effect of the resonator structure on the unloaded quality factor and frequency tuning range is analyzed and simulated. A shorted spiral resonator, with the inner end loaded with a gallium arsenide (GaAs) varactor through air bridges, is chosen for compact size, wide tuning range, and high quality factor. The interstage and external couplings of this resonator remain almost unchanged with the varactor capacitance. Thus, the filter keeps a constant fractional bandwidth in the entire tuning range. The four-pole tunable filter at VHF-band is successfully designed with a compact size of 30 mm x 14.6 mm. The measured results match well with the simulated results. The tuning range of the filter is 17%, from 193.9 to 229.8 MHz. The variation of the fractional bandwidth is within (1.40 +/- 0.01)%. The insertion loss at 229.8 MHz is 0.32 dB, with a reverse bias voltage of 9 V. The unloaded quality factor of the resonators in the four-pole filter is estimated to be 5600 at 229.8 MHz.