Ultra low impedance CPW transmission lines for multilayer MMICs

A novel technique for realizing low-loss coplanar waveguide (CPW) transmission lines with ultralow characteristic impedance on multilayer monolithic microwave integrated circuits (MMICs) is presented. The performance of these CPW lines is investigated experimentally with either ground planes extended under the signal conductor or the center conductor extended under the ground planes. Using this technique, characteristic impedances as low as 7 Omega have been realized. The technique has been compared extensively with the proven standard thin-film microstrip (TFMS) transmission line technique. The TFMS technique can be used for lines with characteristic impedance as low as 3.6 Omega , but is found here to be significantly more lossy. A CPW line with Z/sub 0/=10 Omega gave a loss figure of 0.2 dB per millimeter at 10 GHz, compared to approximately 0.75 dB/mm for the same Z/sub 0/ in a TFMS. These CPW structures are thus expected to find use particularly in uniplanar MMIC power amplifiers.<>