Novel corrugated coupled-line stage with ideal frequency response and its application to bandpass filter design with multi-harinonic suppression

Abstract

Coupled-line stages are designed with corrugated periods to accurately allocate inherent transmission zeros at three leading even harmonics of design frequency, f(0). The corrugation pattern is finely trimmed so that phase velocities of the two eigenmodes can be simultaneously equalized at these frequencies. By decreasing munber of coupled periods in a stage, frequencies of the zeros can be arbitrarily scaled up to a certain extent In this way, a parallel-coupled line bandpass filter with a cascade of several such stages cant be synthesized to be free of spurious harmonics up to at least 10f(0,) with aid of tapped input/output coupling scheme. The measured data of the fabricated bandpass filter show a rejection level of better than 30 dB in the upper stopband up to 18f(0).