A Quantization Noise Reduction Method for Delta-Sigma Fractional-N PLLs Using Cascaded Injection-Locked Oscillators

This paper describes an efficient way to reduce the quantization noise of delta-sigma fractional-N PLLs by using an injection-locked oscillator (ILO) in the feedback path. For further noise reduction, the use of cascaded ILOs is proposed. Behaving like a type-I high-order PLL, the cascaded ILOs effectively suppress the quantization noise like a high-order phase-domain low-pass filter (PDLPF). We show that the proposed ILO-based PDLPF significantly improves the out-of-band phase noise for wideband fractional-N PLLs as well as the in-band phase noise for fractional-N bang-bang PLLs (BBPLLs). Post-layout simulation results show that a ΔΣ fractional-N BBPLL with a 3-stage ILO can achieve the in-band noise reduction of 25 dB without having a nested PLL.