Corrugated parallel-coupled line bandpass filters with multispurious suppression

Corrugated coupled stages are devised to design bandpass filters with multispurious suppression. Through two-port analysis, the suppression is found critically dependent on equality of the even and odd mode phase velocities at even harmonics of design frequency fo. Quarter-wave (l/4) corrugated stages are tuned to allocate inherent transmission zeros at both 2fo and 4fo, and 6fo for some structures, so that the circuit is free of spurious at these frequencies. Coupled stages with proper coupling lengths are arranged to cancel unwanted peaks at 3fo, 6fo and 7fo, and tapped input/output scheme is employed to tackle those at 3fo and 5fo. Measured data of designed filters show rejection levels better than 30 dB in upper stopband. Three circuits are fabricated and measured to demonstrate the idea.