Out-of-band improvement of the quarter wavelength side-coupled ring resonator using low-pass filter integration

Integration of low pass functions into the quarter wavelength side-coupled ring resonator topology is presented as a solution to suppress the parasitic raise in the attenuation band of the resonator response. Two types of 3rd order low-pass filter are proposed: stepped impedance low-pass filter and stub low-pass filter. Using the proposed concepts, three filters were designed and fabricated using microstrip technology on alumina substrate. The experimental results show improvements in the out-of-band response of the resonators.