A compact differential bandpass filter with controllable common-mode transmission zero based on Coupled Cross-Shaped Resonator with short termination

A compact differential bandpass filter (BPF) based on Coupled Cross-Shaped Resonator (CCSR) with short termination is presented in this paper. The differential-mode (DM) and common-mode (CM) equivalent half circuits of proposed CCSR are given and discussed. In DM operation, a wide passband is designed from 4.0 GHz to 5.0 GHz with a fractional bandwidth (FBW) of 22% Under CM condition, controllable transmission zero is realized easily by adjusting the length of short terminated stubs. In addition, the DM responses are unaffected while the CM transmission zero is designed independently. A prototype differential BPF operated at 4.5 GHz with more than 45 dB average CM suppression in DM passband has been realized to validate the proposed concept and theory. The maximum CM rejection is as high as 52 dB due to the controllable zero. The circuit size of this filter is merely 0.12λ₀², showing its compactness advantage.