A Fully Tunable Two-Pole Bandpass Filter

In this letter, a two-pole tunable combline filter that allows for operational agility of the center frequency, passband bandwidth, and locations of upper and lower transmission zeros is proposed. To increase the tunable range of constant absolute-bandwidth for all frequency tuning states, a novel T-type bandwidth-control network was devised between resonators to flexibly tune the coupling coefficient without limitations imposed by the available varactor capacitance. By use of tunable source-load coupling, two tunable transmission zeros were produced on both sides of the passband to significantly improve filter selectivity and to dynamically afford interference suppression. A second-order 1.7-2.7 GHz filter with 1 dB constant bandwidth tuning from 50 to 110 MHz and transmission zero locations tuning from 300 to 700 MHz with respect to the passband was developed and fabricated. Good agreement was obtained between the simulated and experimental results.