A Tunable High Temperature Superconducting Bandpass Filter Realized Using Semiconductor Varactors

This paper presents a semiconductor varactor tuned four-pole bandpass filter. The filter was designed to have a constant fractional bandwidth of 3% and was tuned with single control voltage. The design methods of how to keep coupling coefficients constant and to make all the resonators tuned synchronously are described in details. The filter was built on a two-inch double-side high temperature superconducting film. The center frequency of the filter can be tuned from 430 to 720 MHz with the direct voltage up to 20 V. The return loss is better than 10 dB in the whole tuning range. Insertion loss is in a range of 0.8
$\sim$
3.8 dB with 3 dB fractional bandwidth about 3%.