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

Abstract

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