An Ultra-Compact D-Band SIW Filter with Multifunction Transitions to Coplanar Input/Output

Ultra-compact D-band 3-pole substrate-integrated waveguide (SIW) filters in 50-µm-thick semi-insulating GaAs are designed, modeled, and characterized. At 128 GHz, the filters exhibit a 1.7 dB insertion loss with a return loss > 16 dB and a 3-dB bandwidth of 16 GHz (13%). The compact size of 0.4 mm² is less than half of other D-band SIW filters. The size of the present filter is minimized by enabling the SIW-to-coplanar transition to double as a resonator, and sandwiching between the transitions a double-slot SIW resonator much shorter than the half wavelength with a transmission zero above the pass band. An equivalent circuit model is proposed and verified by both simulation and measurement. The model explains the multifunction behavior of the transition as well as the transmission zero of the double-slot resonator. These results enhance the potential of SIW-based filters for monolithic millimeter-wave integrated circuits.