Varactor tuned ring resonator filter with wide tunable bandwidth

This paper presents a ring resonator bandpass filter (BPF) with tunable passband bandwidth. By varying capacitance of four varactors, mid-upper passband bandwidth can be tuned while mid-lower one is almost fixed. An open stub attached to the ring is designed by using an equivalent serial capacitor, and feed lines are designed by analyzing susceptance slope and return losses. Measured results show that the 3dB-fractional bandwidth (FBW) ranges from 70.1 - 85.3 % with the return loss better than 10.9 dB within the passband.