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

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2 Author(s)
Chuan Ge ; State Key Lab. of Millimeter Waves, Southeast Univ., Nanjing, China ; Xiao-Wei Zhu

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 BW10 dB/BW3 dB 1.74 throughout. The experimental results are in good agreement with theory and simulations.

Published in:

Microwave and Wireless Components Letters, IEEE  (Volume:24 ,  Issue: 7 )