A compact 1.9–2.9 GHz tunable bandpass filter with wide stopband range

This paper presents a compact broadband tunable bandpass filter with wide stopband range. The combline topology is adopted, where the varactor-loaded stepped-impedance resonator is used for frequency tuning. A double-stub parallel-line transformer at input/output port is used for generating multiple transmission zeros and matching port and resonator impedances. The measured performances show that the tuning frequency range of 1.9 to 2.9 GHz (51% tuning range) is achieved. The insertion loss is between 2.9 dB and 1.5 dB and the return loss is better than 18.5 dB. The stopband suppression greater than 20 dB is obtained up to 10 GHz. The circuit design, EM simulation and experimental comparison are described.