Design of a Flat Passband Filter with Tunable Frequency and Bandwidth Using an Integrated Equalizer-Resonator Architecture

In order to achieve a flat passband for the filter, this paper proposes a second-order equalizer with tunable frequency and amplitude, which is integrated with a first-order resonator. To verify that the flat filter can maintain a constant absolute bandwidth across various operating frequencies and achieve different relative bandwidths at the same frequency, the loaded resonator is also designed to be tunable in both frequency and external coupling coefficient. A first-order 1.41–1.61 GHz filter with a flat passband is designed and fabricated, featuring a 1 dB constant bandwidth tunable from 200 to 410 MHz. Good agreement is observed between the simulated and experimental results.