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This brief analyzes the impact of the quantization noise sources in all-digital phase-locked loops (ADPLLs), recently employed as frequency synthesizers. In general, the in-band phase noise is not only caused by the phase quantization of the time-to-digital converter, but also by the frequency quantization of the digitally controlled oscillator (DCO). The delta-sigma modulator placed at the DCO input refines the frequency quantization and adds another source of in-band PLL noise. Interestingly, the higher the modulator order, the higher this source of in-band phase noise. A method for the estimation of all the quantization noise contributors is provided, which is proven by mixed-mode simulations.