Two-pole two-path tunable bandpass filter with multiple transmission zeros

This paper proposes a two-pole two-path mixed coupling tunable filter with a constant bandwidth. A novel frequency dependent I/O structure is presented to generate up to five adjustable transmission zeros (TZs) without increasing the order of the filter. Thus, the selectivity and the out-of-band rejection are enhanced simultaneously in a very compact size. With the coupling characteristic deeply analyzed, a constant bandwidth tunable filter covering from 1.1GHz to 1.38GHz is implemented. The filter has a 3-dB bandwidth of 85±5 MHz and an insertion loss of 1.38-2.1 dB with an excellent performance in selectivity. The experimental results agree with the theory and the simulations very well.