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 f_o. Quarter-wave (λ/4) corrugated stages are tuned to allocate inherent transmission zeros at both 2f_o and 4f_o, and 6f_o 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 3f_o, 6f_o and 7f_o, and tapped input/output scheme is employed to tackle those at 3f_o and 5f_o. Measured data of designed filters show rejection levels better than 30dB in upper stopband. Three circuits are fabricated and measured to demonstrate the idea.