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This paper describes the effect of digital noise on RF circuits on the single chip Bluetooth SoC. Low frequency components in the digital noise, generated by I/O circuits accessing to an external memory, are found to be converted to the phase noise as the spurs of voltage controlled oscillator (VCO). The spurs bring the performance degradation of wireless communications systems. To manage the gain of the VCO and the coupling coefficient is shown to be a key to mitigate the performance degradations.