Wideband High-CMRR Fully Differential Couplers Using Multimode Star-Junction

This article presents novel wideband fully differential hybrid and rat-race couplers whose design methodologies are based on the multimode star junction. The couplers benefit from the even and odd modes of the slotted-microstrip sections that constitute the junction to realize compact equivalent transmission lines, while they exploit the characteristics of the slotline to achieve differential ports with wideband performance and high common mode rejection ratio (CMRR). The differential signals are extracted through slotline to coupled-microstrip line transitions that maintain the phase relation between the ports. Measurements, which are in good agreement with electromagnetic (EM) and circuit simulations, confirm the theoretical predictions. The proposed hybrid coupler has 12% and 333% 15-dB return loss (RL)/isolation and 3-dB fractional bandwidths (FBWs), respectively, 0.6 lambda(g) x 0.6 lambda(g) size, and 0.6-dB insertion loss (IL). While the proposed rat-race coupler has 32.5% and 35% 15-dB RL/isolation and 3-dB FBWs, respectively, 0.7 lambda(g) x 0.7 lambda(g) size, and 0.6-dB IL. For both structures, the CMRR exceeds 50 dB at the operating frequency and the CM rejection FBW exceeds 300%.

Automated summary

This article introduces novel wideband fully differential hybrid and rat-race couplers based on the design methodology of multimode star junction. By utilizing the even and odd modes of slotted-microstrip sections, the couplers achieve compact equivalent transmission lines with wideband performance and high CMRR. Experimental results confirm the theoretical predictions, showing excellent performance of the proposed couplers.