High selectivity and CM suppression microstrip balanced BPF and balanced-to-balanced diplexer

In this paper, a new compact microstrip balanced bandpass filter (BPF) and balanced-to-balanced diplexer are designed with high selectivity and common-mode (CM) suppression. The equivalent differential-mode (DM) and CM half-circuits of the proposed balanced BPF and balanced-to-balanced diplexer are analyzed in detail. Under the DM excitation, the filtering topology is formed by two coupling quarter-wavelength resonators (QWR) and source-load short-ended feedlines. Two transmission zeros can be generated near the DM passband edge to achieve high selectivity. Under the CM excitation, the transmission zero generated by the coupling between the QWR and short-ended feedline can be adjusted by the shorted stub loaded at the center to the DM passband. Moreover, the mutual loading effect of the filters with two coupling uniform resonators is approximately equivalent to a uniform resonator coupling shorted stub. Hence, the mutual loading effect of two balanced BPFs can generate new transmission zeros to deepen the stopband and improve CM suppression without deterioration of high selectivity and CM suppression performance within the DM passband. Finally, the compact balanced BPF and balanced-to-balanced diplexer by combining two balanced BPFs without extra matching network are designed and fabricated, and good agreement can be obtained between the experimental and simulated results.