A Compact Four-Pole Tunable HTS Bandpass Filter at VHF Band

This article presents a compact, low loss, wide tuning range, high temperature superconducting (HTS) bandpass filter at very high frequency (VHF) band. The filter consists of four zigzag open-loop resonators loaded with gallium arsenide (GaAs) varactors as tuning elements. Each resonator is biased independently. The filter circuit is fabricated on a 3-inch HTS thin film deposited on a LaAlO3 substrate. The filter demonstrates a frequency tuning range of 73.2-118 MHz, which is a tunability of 48%, an insertion loss of 0.6-2.2 dB, and a 3-dB fractional bandwidth of 3.2-4.3%. Besides, the return loss of the filter is better than 16.6 dB in the whole tuning range, with slight adjustment of bias voltages for different resonators.