A low cost electrically tunable bandpass filter with constant absolute bandwidth

This paper presents an electrically tunable bandpass filter which provides nearly constant absolute bandwidth (CABW) within the center frequency tuning range. The proposed tunable band pass filter (BPF) is designed on a low cost FR4 substrate with mixed coupled 3rd order open loop ring resonators, high frequency varactor diodes, chip capacitors and some RF chokes. The center frequency of the BPF is tunable from 1.9 GHz to 2.52 GHz with 3 dB absolute bandwidth of 201.5 ± 1.5 MHz. The measured insertion loss of the proposed BPF is found less than 3 dB within the tuning range. There is a good agreement found between the simulated and measured results of the proposed BPF.