Design of compact adaptive rf matching circuits using square split ring resonators

One challenge in the design of an adaptive software defined match control (SDMC) circuits at low RF frequencies is miniaturization. By integrating the SDMC system with the antenna, the impedance of the circuit and input port of the antenna can be matched automatically. Miniaturization is natural at shorter wavelengths (in the millimeter wave bands), yet quite difficult at low RF frequencies (1-2GHz). Previous designs using loaded transmission line stubs, with tuning devices placed on the stubs, have demonstrated excellent tunabilty, but are too large [1]-[4]. Moderate size reduction can be achieved by fabricating the SDMC circuits on high ε_r materials and by folding the stubs. This paper presents the design of SDMC circuits using square split ring resonators (SSRRs). SSRRs are highly coupled transmission line structures that consist of two coupled rings and are electrically small resonators of coupled LC elements. Varactor diodes are placed at the open ends of each coupled ring. Prior works on tunable SSRR structures have focused on adaptive filters that are designed for a 50 Ω input and output impedances. SDMCs, on the other hand, use a 50 Ω input impedance and a frequency dependent output impedance to fulfill the difficult task of matching the reactive part of the antenna impedance. The use of SDMCs with SSRRs requires innovative approaches in order to integrate and isolate the diode bias control circuits. This approach uses vertical interconnects are used to create the bias circuits.