| Summary: | Filters play an invaluable role in most detection and communication systems, blocking unwanted, harmonic or mirrored waves. In this paper, two <inline-formula> <tex-math notation="LaTeX">$3^{\mathrm {rd}}$ </tex-math></inline-formula>-order quasi-elliptical waveguide bandpass filters working at 400 GHz are developed based on TE<sub>102</sub>-mode oversized resonators. Transmission zeros are generated through the physical cross-couplings and the mixed modal bypass couplings. <inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula>-plane off-axis coupling structures without any fragile irises are adopted to strengthen robustness and simplify manufacturing at such high terahertz frequencies. Effects of processing uncertainties on filters performance are discussed in detail to display error sensitivity. Two computer numerical control (CNC) machined filters exhibit a minimum insertion loss of about 1.5 dB in a 3 dB fractional bandwidth (FBW) with 12% centered at 390 GHz @ Filter-I, and 9% FBW with a center frequency of 394 GHz @ Filter-II, which are all mainly agreement with simulations. The performance is highlighted with the reported terahertz filters, which indicates that both architectural designs are still suited for the delicate CNC-milling restriction in such high WR-2.2 band.
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