A Silicon Monolithic Phase-Inverter Rat-Race Coupler Using Spiral Coplanar Striplines and Its Application in a Broadband Gilbert Mixer

This paper realized a broadband uniplanar phase-inverter rat-race coupler using a standard silicon process, and then analyzed this coupler under a lossy condition. A phase inverter is employed in this coupler, not only to extend the operation bandwidth, but also to generate balanced outputs by providing equal lossy paths, while symmetrical spiral-shaped coplanar striplines (CPSs) are also utilized to shrink the coupler size, as well as to construct a phase inverter in the middle of one of spiral CPSs. The lossy CPS, when designed as a distortionless line, has a real characteristic impedance, and thus, perfect port matching of the coupler can be achieved at the center frequency. The operation frequency of this silicon monolithic rat-race coupler with the size of 0.5 mm² is extremely wide and ranges from 5 to 23 GHz. The dissipated loss, transmission coefficient, and isolation of the rat- race coupler are approximately 5.5, -8, and below -25 dB, respectively. In addition, a wideband Gilbert micromixer with an integrated uniplanar phase-inverter rat-race coupler at the local oscillator port is demonstrated using 0.35-mum SiGe BiCMOS technology. This mixer works from 2.5 to 13 GHz with 12-dB conversion gain, -16-dBm IP_{1 dB}, -4-dBm IIP₃, and 14-dB noise figure. The chip size of the mixer with an integrated coupler is approximately 1.4 mm times 1.4 mm.