Short-Ended Self-Coupled Ring Resonator and Its Application for Balanced Filter Design

Theory and applications of short-ended self-coupled ring resonator (SCRR) are focused in this letter. By periodically and symmetrically placing short-ended SCRRs between the microstrip differential lines, the common-mode (CM) suppression can be achieved at desired frequency easily with wideband unaffected differential-mode (DM) passband. On the basis of controllable center frequency of CM suppression and cut frequency of DM passband by means of self-coupled coefficient, differential lines with SCRRs and a balanced bandpass filter (BPF) are designed at 2.4 GHz. Due to the effects of the proposed short-ended SCRRs, the balanced BPF exhibits more than 60 dB of maximum CM rejection within the DM passband. The simulated and measured results are in good agreement.