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In many wireless systems, the transmit (TX) signal could leak to the receiver input through the antenna duplexer, and this leakage could raise the noise level in the receive (RX) band. Therefore, it is important to be able to accurately predict the noise spectrum that leaks into the RX band from a TX digital-to-analog converter (DAC) in order to optimize the noise performance. This study analyzes the DAC output noise that is caused by the phase noise of the DAC clock. First, a closed-form equation is derived to account for both the phase noise and noise aliasing. Then, the proposed TX leakage prediction in the RX band is verified with both simulation and measurement. A clear understanding of the relationship between the clock phase noise and the DAC output noise spectrum could enable both the DAC and phase-locked loop designs to be optimized for the lowest RX band noise performance with low cost and power consumption at the same time.