A tunable transformer-based directional coupler for UHF RFID readers is presented. Based on the current cancellation between inductively and capacitively coupled power through a non-inverting transformer and two coupling capacitors, respectively, the proposed directional coupler exhibits high isolation performance. In addition, by a tuning capacitor array and a terminal resistor array, the notch frequency of the coupler is tunable over the global UHF RFID band. The coupler is fabricated in a

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... C 0.18-µm CMOS process and the measured isolation is better than −60-dB from 840 MHz to 960 MHz. Besides, the transmission loss and the coupling factor at the notch frequency vary from −0.86-dB to −0.7-dB and from −16.2-dB to −15.3-dB, respectively.