AbstractThis paper presents an ultra-wideband coplanar waveguide (CPW)-to-asymmetric coplanar stripline (ACPS) transition based on aluminum nitride (AlN) substrate. The concepts of designing CPW, ACPS, and CPW-to-ACPS transition are explained. In order to suppress parasitic modes, vias going through AlN substrate are added along the ground traces. The signal trace is tapered out and chamfered to reduce the reflection caused by the termination of ground trace. The CPW-to-ACPS transition is designed, fabricated, and measured in a back-to-back configuration. The fabricated CPW-to-ACPS transition can provide a bandwidth of 165 GHz with an associated insertion loss of 3 dB.