A tunable bandpass filter with constant absolute bandwidth based on one ring resonator

A miniaturized tunable bandpass filter (BPF) with constant absolute bandwidth (ABW) based on a single ring resonator is presented in this paper. The proposed BPF is composed of one ring resonator and a pair of 1/4 lambda resonators loaded with varactors. The working frequency of the BPF depends on the pair of 1/4 lambda resonators. The ring resonator is introduced to transfer energy effectively and improve the characteristics of the BPF. An experimental filter has been designed and fabricated which maintains constant ABW across the tuning range from 1.80 to 2.44 GHz. The predicted results are compared with measured data, and a good agreement is reported, which verifies the proposed method.