Analysis and Design of On-Chip High-Selectivity Sub-THz Bandpass Filter Using Dual-Mode Resonators in 65 nm CMOS Technology

A compact sub-THz bandpass filter (BPF) implemented in 65 nm CMOS technology is presented. By exploiting a stub-loaded dual-mode resonator (SLR), the design achieves enhanced frequency selectivity without expanding the physical circuit area. An LC circuit model of the SLR is proposed, demonstrating its capability to implement a higher-order transfer function with additional transmission zeros (TZs). To further narrow the transition band, a new cross-coupled BPF topology based on the SLR is then proposed in which two TZs are placed inside an initial passband to define the final band edges. This produces sharp roll-off at the band edges. The cross-coupling itself reuses the SLR arms, reducing the area cost typical of cross-coupled topologies. A systematic design procedure is provided to guide the synthesis. The measured results demonstrate that the filter operates at a center frequency of 178 GHz with an insertion loss of 5.0 dB and a fractional bandwidth of 25%. The filter achieves a shape factor of 0.51, occupying a core chip area of only 0.129 × 0.273 mm2 (0.0124λ02).