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

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.