Charge-Pump Folded Noise Cancelation in Fractional-N Phase-Locked Loops

In this brief, we propose a technique for Delta-Sigma (ΔΣ) noise cancelation in fractional-N phase-locked loops (PLLs). The proposed method performs noise cancelation for both the shaped noise generated by the ΔΣ modulator and the downfolded noise due to charge pump (CP) nonlinearity. By precoding the CP control pulses, the added hardware complexity is minimal compared with conventional cancelation, whereas noise reduction is large. The proposed method is verified through the system design of a 2.5-GHz ΔΣ fractional-N PLL. Time-domain simulations show 20-dB noise reduction for the downfolded noise when applying the proposed cancelation.