Tunable Dual-Band Filter and Diplexer Based on Folded Open Loop Ring Resonators

Tunable dual-band filter and diplexer based on folded open loop ring resonators (OLRRs) are proposed in this brief. Two pairs of OLRRs of different center frequencies are utilized to couple with microstrip transmission lines in these two devices, and there is no extra matching network in the diplexer structure. Maximal magnetic coupling and high isolation can be obtained by placing the resonators at proper positions along the feeding lines. The passband/channel frequencies of the dual-band filter/diplexer can be tuned independently by varying dc bias voltages applied to the varactors loaded at one end of the resonators. In addition, a broad harmonic suppression can be obtained by choosing appropriate lumped elements adding to the OLRRs. The performances of the proposed devices can be verified by the simulated and measured results.