Coplanar branch-line and rat-race couplers for W-band applications

In this paper, we report on coplanar branch-line and rat-race couplers on GaAs for use in balanced mixers, investigated theoretically and experimentally over the frequency range from DC to 120 GHz. For optimum spurious response suppression at the IF-port of a balanced mixer, a coupler must provide the required phase delay at its ports feeding the non-linear devices. Whereas in microstrip (MS) technology these types of couplers have been extensively investigated in the past, coplanar waveguide (CPW) technology is still lacking similar broadband models, making millimetre-wave circuit design difficult. For the characteristic impedance levels 30, 50 and 67 Ω, a model library was developed for transmission lines and line elements like air bridges and corners, and implemented into HP-MDS. On the basis of these models, we designed a branch-line coupler, a capacitively loaded branch-line coupler and a rat-race coupler for W-band applications. For the analysis of the phase relation between the different ports of these hybrids, 2-port test structures were designed and processed. The 'on-wafer' measured S-parameters of the couplers show excellent agreement with the simulated data in both amplitude and phase over the entire frequency range.