Tunable inter-resonator coupling structure with positive and negative values and its application to the Field-Programmable Filter Array (FPFA)

In this paper, we show a tunable inter-resonator coupling structure capable of varying the coupling coefficient between resonators from positive to negative values, including values that approach zero. The presented inter-resonator coupling structure can be tuned to generate large isolation between two resonators as well as the required coupling for filter responses. Using the inter-resonator coupling structure, this paper demonstrates the concept of a field-programmable filter array (FPFA). The proposed array is composed of tunable resonators and can have multiple functionalities by routing signals from input ports to output ports. Signal routing can be achieved by controlling the inter-resonator coupling with a wide tuning ratio of coupling coefficients. A unit cell of the proposed FPFA was fabricated to prove the proposed concept. It is shown that the unit cell can be adjusted to have filter array responses with two second-order bandpass responses, third-order bandpass responses, and fourth-order bandpass responses. The unit cell can also be operated as a switchable filter bank without a switch. All operation modes are verified by measurements. This paper also demonstrates, for the first time, a reconfigurable filter that can be tuned to have both elliptic and self-equalized responses using the presented inter-resonator coupling structure.