An Area-Efficient Reconfigurable Compact Multi-Band Directional Coupler in RF SOI CMOS Technology

In this paper, a reconfigurable directional coupler covering a wide range of operating frequency is presented. Excessive insertion loss at high frequencies of an inherently high-pass weakly coupled electrically short coupler limits the range of operating frequencies. In the proposed design, the effective coupling strength between the primary and pick-up coils can be switched, thus reducing the coupling coefficient at higher frequencies. An area-efficient physical design is adopted by reusing the area inside the primary coil, occupying a core area of $0.072 \mathbf{m m}^{2}$. The device is implemented in a 90 nm RF SOI CMOS switch technology. The designed directional coupler shows a coupling coefficient between 25 dB and 20 dB from 900 MHz to $2.8 \mathbf{G H z}$. Insertion loss at $2.8 \mathbf{G H z}$ is improved from $0.253 \mathbf{d B}$ to $0.185 \mathbf{d B}$. Directivity of more than $25 \mathbf{d B}$ can be reached from $700 \mathbf{~ M H z}$ to $3.25 \mathbf{~ G H z}$ with the on-chip matching termination.