Spur reduction by self-injection loop in a fractional-N PLL

This paper presents a method for spur noise reduction in a fractional-N PLL. First, we applied our proposed model of the response to an external injection in oscillators to the liner model of the PLL. It was found that the self-injection that injects an appropriately delayed oscillator output into its own oscillator could reduce the current fluctuation caused by operation of the fractional-N PLL. We designed and fabricated a testchip to verify our analysis in a 0.18-pm CMOS process. We examined the effect of the self-injection by applying the injection to a simple fractional-N PLL while adjusting the timing and the strength of the injection. According to the measurement results, the level of the fractional spur can be reduced by 1.5 dB. This result indicates effective control of the PLL loop bandwidth by self-injection.