An analysis of miniaturized dual-mode bandpass filter structure using shunt-capacitance perturbation

A dual-mode bandpass ring filter using two pairs of shunt capacitances has been analyzed and designed. The size of the ring resonator can be reduced significantly by increasing the shunt capacitances. Since the resonator size is not an integer multiple of operating frequency, harmonic suppression is easily achieved. Analysis shows that the two pairs of shunt capacitances independently control the even- and odd-mode resonant frequency of the ring resonator. Moreover, the shunt capacitance also allows easier biasing, making tunable filters using varactors realizable. Various configurations of dual-mode bandpass filters have been reported, and the advantages and disadvantages of each structure are discussed. Several filters of various capacitances have been designed and tested at 1.8 GHz, and show significant size reduction of over 67%. A varactor-tuned filter has also been designed, and demonstrated a measured tunable center frequency of 20%. This study is a solution to the miniaturization of ring filters, and allows performance tuning, making ring filters more attractive in monolithic microwave integrated circuits and system-on-chip applications.