Compact CMOS branch-line coupler using high-Q slow-wave transmission lines

This article presents a miniaturized 90 degrees coupler incorporating slotted conductor-backed coplanar waveguide (S-CBCPW) transmission lines (TLs) and four shunt capacitors. The periodically slotted ground achieves higher slow-wave factor (SWF) or effective permittivity (epsilon eff), and lower attenuation constant. Electrical characteristics of the S-CBCPW TLs are investigated by different slotted ground layers. Moreover, trade-offs among SWF, characteristic impedance, and electrical length for the design are also discussed. Finally, the measured 24-GHz coupler not only features smallest normalized chip area among previously reported CMOS couplers but also demonstrates excellent results which are comparable to the designs at 60 GHz. (c) 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:11741178, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27525