Hybrid-ring-based signal-interference planar lowpass filters with ultra-broad stopband

A new approach of microwave planar quasi-elliptic-type lowpass filter (LPF) with ultra-wide stopband is presented. It makes use of an original class of bandstop transversal filtering section (TFS) based on a generalized version of the hybrid ring. This circuit configuration, operating under feedforward signal-combination principles, enables sharp-rejection lowpass transfer functions with multiple transmission zeros (TZs) to be generated. The flexibility of the devised bandstop TFS to control its electrical performances is verified. Also, two proof-of-concept multi-stage LPF microstrip prototypes are developed and measured. They are ideally designed to exhibit a 1-GHz-3-dB-cut-off-frequency passband with a minimum power matching level of 20 dB and a broad stopband for a power rejection capability higher than 20 and 40 dB, respectively.