Skip to Main Content
This paper describes a noise filtering method for Â¿Â¿ fractional- N PLL clock generators to reduce out-of-band phase noise and improve short-term jitter performance. Use of a low-cost ring VCO mandates a wideband PLL design and complicates filtering out high-frequency quantization noise from the Â¿Â¿ modulator. A hybrid finite impulse response (FIR) filtering technique based on a semidigital approach enables low-OSR Â¿Â¿ modulation with robust quantization noise reduction despite circuit mismatch and nonlinearity. A prototype 1-GHz Â¿Â¿ fractional-N PLL is implemented in 0.18 Â¿m CMOS. Experimental results show that the proposed semidigital method effectively suppresses the out-of-band quantization noise, resulting in nearly 30% reduction in short-term jitter.