A novel compact capacitive loaded differential bandpass filter

A novel compact differential bandpass filter (BPF) with parallel coupled resonator and capacitive load is proposed in this paper. The differential- and common-mode equivalent half circuits are given and discussed. By properly designing the coupled resonator and the loaded capacitance, differential-mode passband responses and common-mode rejection can be obtained. In common-mode operation, loaded capacitive network offers wideband attenuation response suppressing common-mode signal. A prototype differential BPF operated at 4.2 GHz with more than 20 dB common-mode suppression from 0 to 9 GHz has been realized to validate the proposed concept and theory. In addition, the circuit size of this filter is merely 0.09λ₀², showing its compactness advantage.