Series coupled microwave ring resonators

An odd number of one-wavelength ring resonators are proposed to be directly series-connected along a quarter-wavelength of their sides to exhibit a bandpass response with very high rejection level. As each ring presents a dual-mode response, the order of the overall bandpass response will be equal to two times the number of rings being used. Furthermore, the pseudo-elliptic characteristics of the ring resonators will enhance the selectivity of the resulting filter since a transmission zero is obtained at both sides of the passband. An application of the concept is implemented for the case of three series-connected rings that exhibits a 6th order bandpass response centered at 2 GHz. Fabricated on epoxy-glass substrate, the results show a very good agreement between the simulation and measurement with a very good rejection level surpassing 50 dB up to 3.5 GHz.