Quasi-Planar Compact S-Band Quadrature Coupler Using MEFAB-Based Two-Stage Weak-Coupling Transformer

This letter presents a design method of a quasi-planar compact quadrature coupler based on micro-structure electrochemical fabrication (MEFAB) process. Two S-band quadrature couplers based on a two-stage weak-coupling transformer with different capacitors were designed, maintaining a phase difference of$90^{\circ } ~\pm ~3^{\circ }$across their operating bands. The quadrature coupler with lumped-capacitors operates over a bandwidth 2.26–2.94 GHz with a circuit size of$0.0053\lambda $g2. Therefore, the quadrature coupler with metal–insulator–metal (MIM)-integrated capacitors achieves a bandwidth of 2.41–3.59 GHz with a circuit size of$0.0052\lambda _{\mathrm {g}}^{2}$.