Ultra-Narrowband Silicon-Micromachined Sub-THz Filter With Wide Spurious-Free Rejection Band Employing High-Q TM 330 Resonators

In this article, we present an ultra-narrowband silicon-micromachined bandpass filter with a wide and high-rejection stopband. The proposed filter uses high- $Q$ factor TM330 mode resonators. To avoid near-passband spurious resonances typically associated with higher order modes, a novel method of arranging the positions of the coupling slots is carried out. A fourth-order filter with a center frequency of 183 GHz and a fractional bandwidth (FBW) of 0.5% has been fabricated by silicon micromachining for the first time. The prototype employs out-of-plane transitions on the input and output ports, which results in axial ports enabling a direct characterization with the device simply mounted between the two standard waveguide test ports. The unloaded $Q$ factor extracted from the measurements is 1000, which is unparalleled by any previously reported narrowband filter in any technology in this frequency range. A spurious free response with a high stopband rejection in the entire waveguide band is obtained. The measured worst-case insertion loss and return loss (RL) in the passband are 4.5 and 9 dB, respectively.