Design of a monolithic 30 GHz branch line coupler in SiGe HBT technology using 3D EM simulation

We use full 3D EM simulation to optimize the design of a 30 GHz branch line coupler using thin film microstrip lines (TFMS) in a commercial 120 GHz SiGe HBT BiCMOS technology. The effects of various ground plane connections are considered for designing the branch line coupler. With an improper design of the ground plane in the TFMS, the measured S/sub 21/ shows 7 dB degradation at 30 GHz, in agreement with simulation. A carefully optimized coupler design shows that -4 dB S/sub 21//S/sub 31/ with -15 dB reflection coefficient at 30 GHz should be achievable.