A Novel Tri-Mode Bandwidth Tunable Filter with Harmonic Suppression

In this paper, a novel bandwidth reconfigurable bandpass filter is proposed. Based on a varactor loaded tri-mode resonator consisted of one constant odd mode and two independent varactor-tuned even modes, each passband edge of the proposed filter can be freely adjusted. By varying the reverse bias voltage applied to the varactor diode that is connected to the resonator, the bandwidth can be controlled conveniently. The resonant frequencies and Transmission Zeros (TZs) are derived and verified by both theoretical analysis and simulation. Stepped Impedance Resonator (SIR) is introduced to optimize the harmonic suppression. Finally, the measurement of the fabricated filter shows a fractional bandwidth tuning range of 11.4-32.0 % with a center frequency of 1.75 GHz, a quite low insertion loss of 0.4-1.1 dB, and wideband harmonic suppression up to 6 GHz. The measurement and simulated results show good agreement.