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This paper describes a circuit design methodology to optimize both power and jitter performances of the inductorless voltage-controlled oscillators (VCOs) by utilizing an on-chip supply noise monitoring circuit. As the phase-locked loop (PLL) has a sensitive response to the noise frequency near the loop bandwidth, detecting low frequency tones up to the PLL bandwidth is considered for a supply noise monitoring circuit, which makes it possible to design a low cost semi-digital noise monitoring system. This work affirms the importance of optimizing the voltage swing amplitude of the inductorless VCO, which sets a fundamental trade-off between low power and low noise performances. Transistor level simulations are done to verify the effectiveness of the proposed method for the ring VCO and the relaxation VCO.