Superconducting varactor tunable filter with constant bandwidth using coupling line

This letter presents a superconducting tunable filter with a constant bandwidth using semiconductor varactors. A coupling line is introduced to adjust the inter-resonator coupling to meet the requirement of constant bandwidth. The external couplings are designed to remain a good passband shape with the tunable frequency. With the proposed method, a superconducting tunable filter with constant bandwidth is successfully implemented. The measured results of the filter agree well with the simulated ones. It shows a frequency tuning range of 11% from 924.5 MHz to 1030.8 MHz, a 1 dB bandwidth of 23.8 ± 0.3 MHz, and an insertion loss of 0.16 to 1.41 dB.