A Compact Fourth-Order Tunable Bandpass Filter Based on Varactor-Loaded Step-Impedance Resonators

In this paper, a compact high-selectivity frequency tunable bandpass filter (BPF) with constant absolute bandwidth (ABW) based on varactor-loaded step-impedance resonators (SIRs) is presented. By introducing cross coupling between resonators, pair of transmission zeros (TZs) close to the passband are produced and selectivity enhanced significantly. Another TZs generated improve out-of-band rejection by using source-load coupling. The SIRs utilized design fourth-order BPF in order realize wide tuning range size. addition, frequency-dependent feeding structures employed instead lumped capacitors used conventional structures, as result, insertion-loss performance improved. simulated measured results presented show good agreement. exhibit from 0.8 1.14 GHz 3 dB ABW about 47 ± 5 MHz, return loss greater than 13.9 dB, insertion 2.7–3.1 dB. Moreover, four generated, proposed shows high rectangular coefficient 2.3–3.1.