At UHF to lower microwave frequencies, RF front-end circuits are primarily made by monolithic microwave integrated circuits (MMIC). The typical MMIC makes use of microstrip line technologies for which sufficiently accurate and fast CAD software is available, and industry all over the world has sufficient design experience. At higher microwave frequencies to millimeter wave frequencies beyond 60 GHz, a microstrip line is not necessarily the best choice, while coplanar waveguide (CPW) technology has received considerable attention. At this time, CAD tools for CPW are not completely satisfactory. CPW has much smaller dispersion in phase velocity and has a more tightly coupled electromagnetic guided field than microstrips and many other guided wave structures. In addition, its uniplanar nature does not require a via-hole process due to lack of the backside ground plane while air-bridges are required. IMST in Germany has spent considerable effort in establishing design techniques for CPW, primarily based on lumped element approximations. A number of corporations visited by the WTEC panel, including Daimler-Chrysler and NEC (see Fig. 5.14), now make use of CPW for millimeter-wave MMIC development.
Fig. 5.14. NEC 60 GHz CPW MMIC.
Use of CPW and slot line in MMIC have been core research items at NTT Wireless Systems Laboratory. Its Uniplanar MMIC can reduce the chip size to 1/1.5 to 1/5 in comparison with the microstrip line based MMIC. NTT has further advanced research toward 3D MMIC, which is discussed later in this chapter.