Series-cascaded single-side-coupled ring resonators

A second- and a third-order filters are presented based on the single-side-coupled ring resonator topology. A simple way to obtain the second-order bandpass response is proposed by adding a quarter-wavelength coupled line immediately at the coupled-line side of the resonator, which forms a half-wavelength resonator, hence an extra pole. Using the similar idea, an extra pole is obtained from a half-wavelength resonator that is formed between two identical resonators cascaded in series. Both filters are designed, fabricated and measured to show the feasibility of the idea, on FR4 substrate using microstrip.