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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.072 \mathbf{m m}^{2}$</tex>. 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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.8 \mathbf{G H z}$</tex>. Insertion loss at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.8 \mathbf{G H z}$</tex> is improved from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.253 \mathbf{d B}$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.185 \mathbf{d B}$</tex>. Directivity of more than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$25 \mathbf{d B}$</tex> can be reached from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$700 \mathbf{~ M H z}$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3.25 \mathbf{~ G H z}$</tex> with the on-chip matching termination.