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

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 > 16 dB return loss and a 3-dB bandwidth of 16 GHz (12.5%). The compact size of 0.4 mm² is less than half of other D-band SIW filters. The size of the 3-pole SIW filter is minimized by using the SIW-to-coplanar transitions as mode transformer and resonators, and a two-slot SIW resonator that introduces a transmission zero above the bandwidth. An equivalent circuit model is proposed and verified by simulation and measurement. The model explains the multifunction behavior of the transitions as well as the transmission zero from the two-slot resonator. These results indicate the potential of SIW-based filters for millimeter-wave integrated circuits.