Compact Tunable Bandpass Filter With a Fixed Out-of-Band Rejection Based on Hilbert Fractals

This paper proposes a new type of compact tunable bandpass filter with bandwidth tuning and out-of-band fixed rejection. Because we employ the modified Hilbert fractal structure loaded with varactors as resonators, the tunable filter has a very compact configuration and a constant shape over the entire tuning range. The frequency selectivity is improved by introducing a cross coupling between the source and the load. As a result of the utilization of a pair of properly designed feedlines, the frequency tuning and the out-of-band rejection of the filter are independent of each other, which simplifies its operation significantly. Two filter prototypes have been realized with the same size of 25.0 × 17.0 × 1.0 mm³. Their superior performances have been demonstrated experimentally, with good agreement obtained between their simulated and measured S-parameters.