Miniaturized rat-race coupler with out-of-band suppression using double-layer synthesized coplanar waveguides

We propose and investigate a new miniaturized rat-race coupler with out-of-band suppression capability. The new design consists of six synthesized coplanar waveguide (CPW) cells, formed by meander line inductors, parallel-plate capacitors, and interdigital capacitors. The coupler, with a fractional bandwidth of 13%, is merely 6.2% the size of its conventional counterpart. Benefitting from the embedded parallel LC tank in the new synthesized cell, the coupler shows signal suppression capability from 1.75 to 2.45 GHz at the same time. The rejection level is greater than 25 dB at the first harmonic (1.8 GHz).