Microstrip side-coupled ring bandpass filters with mode coupling control for harmonic suppression

A technique to suppress higher harmonics in side-coupled ring bandpass filters is proposed and demonstrated. The proposed technique basically consists of optimising the ring shape and the excitation stage of the complete filter structure in order to mitigate the excitation of the resonant modes that generate the second-harmonic filter resonances. A harmonic suppression ratio for the second harmonic greater than 25 dB in a flat bandpass filter centred at 2.5 GHz is experimentally demonstrated.