Planar microstrip ring resonator filters

Describes tunable ring resonator filter elements, with potential applications in personal communications systems. The simple microstrip ring resonator consists of a closed loop of microstrip. Such resonators have lower Q factors than dielectric puck or YIG resonators but they are adequate for many filter applications, and have a particular advantage in the context of personal communications in that they are compatible with surface mount technology. In the typical case when the line width is small compared to the radius, the resonant modes can be determined by simple transmission line theory. Ring resonators can be made tunable by mounting variable capacitance diodes across gaps in the ring, or to ground from suitable points on the ring. Tuning curves for several configurations are discussed. Theoretical and practical results are given for a 1.8 GHz element. Low-cost MMIC amplifiers are incorporated to compensate for resonator losses, and novel features are included to reduce higher frequency resonances. The active devices are mounted inside the ring to minimize the size