Superconducting sub-millimeter diplexer suitable for pixel-size two-band bolometric detection

The design of a diplexer, coupled to a planar bow- tie antenna for two-band cosmic microwave background detection is proposed, using both HFSStrade and ADStrade softwares. The two photometric bands are centered on 150 GHz and 220 GHz respectively. The diplexer is composed of two three-elements distributed filtering circuits, taking into account the complex impedance of the antenna and the impedance of the two resistive loading bolometers. The distributed elements are transmission lines coplanar strip lines (CPS) and broadside coupled lines (BCL). Their balanced geometry matches to the differential signal delivered by the bow-tie antenna, avoiding the addition of a balun circuit. The envisaged technology for strips and antenna is superconducting niobium at a few 100 mK on Si substrate. The overall size of the antenna/diplexer structure is around 2 mm², allowing to consider it as a pixel suitable for integration in a bolometer array.