Highly-Selective Tunable Bandpass Filter With Two-Path Mixed Coupling

This letter proposes a two-path mixed coupling tunable bandpass filter with constant bandwidth. This compact configuration can achieve little bandwidth variation in a large tuning range. Presented with the equivalent circuit of the coupling structure, the analytical formulas of the two-path mixed coupling structure are derived to determine the initial dimensions of the resonator quickly. Furthermore, to enhance the selectivity and the out-of-band rejection, additional resonance circuit is introduced in the I/O structure to generate multiple adjustable transmission zeros. For verification, a tunable filter covering the frequency band of 0.78-1.35 GHz is implemented and measured. The filter has a 3 dB bandwidth variation of only 5.8% in the tuning range of 54.2%, an insertion loss of 1.68-2.9 dB and measured BW_{10 dB}/BW_{3 dB} 1.74 throughout. The experimental results are in good agreement with theory and simulations.