Inline Waveguide Filters With Transmission Zeros Using Frequency-Variant Couplings

This article introduces a novel frequency-variant coupling structure with high adjustability for waveguide filters. Although this structure is proposed for rectangular cavities, the same principle can be applied to other cavity geometries. The proposed structure is compatible with standard micromachining technologies. Using this coupling structure, two inline Nth-order waveguide filters with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N-1$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N+1$ </tex-math></inline-formula> transmission zeros (TZs) are developed. A fourth-order bandpass filter (BPF) with three TZs at 245, 285, and 288 GHz is designed, operating at a center frequency of 270 GHz with a 2% fractional bandwidth. In addition, a second-order BPF with a center frequency of 280 GHz and a 0.71% fractional bandwidth, utilizing three TZs at 230, 300, and 308 GHz, is also developed. Prototypes of both filters are fabricated using silicon micromachining. The measured insertion loss and return loss (RL) in the passband are 2 dB and better than 18 dB, respectively, for the fourth-order filter and 2.1 dB and better than 11 dB for the second-order filter, respectively.