A constant absolute bandwidth tunable filter using varactor-loaded open-loop resonators

This paper presents a new type of lager tuning rate tunable filters using varactor-loaded open-loop resonators. The filter is based on capacitively loaded folded λ/2 resonators that are coupled to each other with mixed electrical and magnetic coupling. By adjusting the value of the varactors between the open gaps in a mixed coupled filter, the coupling coefficients and external quality can be tuned to meet the requirement of constant absolute bandwidth. The fabricated filter shows a tuning rate of 460 MHz/pF. To our knowledge, this planar tunable filter represents state-of-the art tuning rate characteristics among the varactor-tuned filter.