Miniature CMOS Stacked Spiral-Coupled Directional Coupler With $-$67-dB Isolation and $-$ 0.8-dB Insertion Loss

This letter proposes a miniature CMOS stacked spiral-coupled (SSC) directional coupler for a fully integrated CMOS transceiver to isolate the large transmit power from the transmitter to the receiver. It consists of two 90 °-rotated center-aligned SSC coils. The path center of one coil is aligned to the gap center of the other coil. The grounded secondary coil plays a role of ground shielding for reducing the insertion loss. Two capacitors are utilized for tuning the center frequency and isolation level. Fabricated in a standard CMOS process, it achieves an isolation of -67 dB, an insertion loss of -0.8 dB, and a coupling of -16.1 dB and only occupies 0.3×0.3 mm2 of silicon area. To the authors' knowledge, this is the first CMOS directional coupler at the UHF band, with the smallest form factor when compared with other implementations of directional couplers.