High-Selectivity Tunable Bandpass Filters With Harmonic Suppression

This paper presents harmonic-suppressed tunable bandpass filters with two movable transmission zeros. For tunable bandpass filters, tuning the passband frequency will cause the harmonic to shift, complicating the harmonic suppression. To overcome this problem, lumped elements are utilized to realize harmonic suppression without degrading passband performance. It is found from theoretical analysis that at even-order harmonic frequencies, the lumped elements could not only decrease the resonator Q and dissipate RF power, but also control even-order impedance and cause mismatching at filter input/output ports. Both of the factors help reject even-order harmonics. Meanwhile, the features at fundamental resonant frequencies are nearly not affected by these elements, indicating harmonic suppression could be achieved without affecting passband performance. This property is experimentally verified by comparing the responses of tunable bandpass filters with and without harmonic suppression. Finally, a harmonic-suppressed tunable bandpass filter with constant bandwidth and passband shape is designed. A novel input and output coupling structure with a bandpass response is employed to maintain constant bandwidth and help reject both even- and odd-order harmonics. For each tuning state, two transmission zeros are created near the passband, ensuring high selectivity.