High common‐mode rejection balanced bandpass filter using novel coupling scheme and source‐loaded coupling technique

Abstract

A balanced bandpass filter (BPF) with a novel coupling scheme and source‐loaded coupling (SLC) technique is proposed. The J‐inverter is directly connected to the source or load, and the single‐band cell (SBC) is fed by the J‐inverter. Meanwhile, the J‐inverter is also coupled to a resonator. Equivalent differential‐mode (DM) and common‐mode (CM) circuits are presented, and their ideal frequency responses under DM and CM are also provided. Furthermore, the SBC equals paralleled short‐ and open‐circuited stubs in DM and equals two paralleled open stubs in CM. To validate the design concept, a four‐pole balanced BPF working at 2.66 GHz with fractional bandwidth (FBW) of 13.8% is designed and fabricated. Three transmission zeros (TZs) are generated, and the upper stopband suppression level reaches 34 dB. Meanwhile, a high CM rejection level attains 54 dB within the DM passband.