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A broad.band transition fmm a coaxial line to a CPW chip is simulated and realized. It is part of a 80 Gbps optoelectronic communication system covering the frequency range up to 120 GHz. The extremely large bandwidth require efforls not only to keep reflections low but also to suppress unwanted modes. I. INTRODllCTlONWith increasing hit rates in optoelectronic communication systems, proper interconnects are to be realized in order to connect the chip to the outer world. To tackle the very high speed data exchange, a bmad-band approach is inevitable providing low reflections and insertion loss as well 3s high immunity to package resonances. For 40 Gbps applications, the well-known approaches using BGA's, vias or bond wires result in a sufficient transmission behavior [1][2][3][4]. For hi@er frequencies, BGA arrays cannot he arranged dense enough and the bond wire inductance is prohibitively large. Other methods [51 use coaxial-to-microsrrip transitions with SMA connectors, which are a method of choice for 10 Gbps applications hut not applicable beyond 25 GHr.In our work, the frequency range up to I20 GI-lz is covered. This demands the connecting elements between CD axial line and CPW on the chip to be small in size and to show only low discontinuity effects. The design of such a transition requires a thorough investigation of the dimensions and material properties of the CPW chip, the transition and the carrier substrate. The main task is to achieve a transition from coaxial mode into CPW mode with low insertion loss and reflecrions, while the parasitic modes generated are suppressed as much as possible.This paper presents a detailed investigation on the t msition, its electromagnetic properties, and the optimization concept. The results are verified by measurements.11. TkLE STRUCTURAL OVERVIEW Fig. 1 shows the CPW-to-coax transition. The interconnect is realized by so-called pre-shaped metal lids (PSLs), which consist of 15 pm thick Au and are bonded 485 0-7803~8331-