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We present a new all-digital RF in-phase/quadrature (I/Q) modulator, in which the orthogonal summing of the I and Q phase data signals is performed in separated interleaved time slots. By employing a 25% duty cycle for the I and Q signals, the modulator can directly reconstruct the continuous-time RF output signal using four digital switch arrays with a power combiner. To verify the proposed concept and its related design procedure, a 65-nm CMOS prototype is implemented. This prototype achieves 12.6-dBm peak output power with 20% peak drain efficiency at 2 GHz. The corresponding error vector magnitude (EVM) for a quadrature phase-shift keying constellation is 3.95% without any predistortion, while providing 6-dBm output power in a 64 quadrature amplitude modulation constellation with a related EVM and drain efficiency of 2.36% and 10%, respectively. The proposed circuit can be used as a pre-driver or a final transmit stage. The first-ever truly all-digital I/Q RF digital-to-analog converter prototype is thus experimentally demonstrated.