19.5 A 3.2GHz digital phase-locked loop with background supply-noise cancellation

Abstract

Phase-locked loops (PLLs) are widely used in various applications such as processors, consumer electronics, and wireline communication systems. When digital circuits and a PLL with a ring oscillator are integrated together, the power supply noise may degrade the jitter performance of the PLL. To lower the supply-noise sensitivity of a PLL, several approaches [1-5] have been proposed. A passive decoupling capacitor and/or a low-dropout (LDO) regulator [1] can be used to suppress the supply noise. However, the decoupling capacitor occupies large area and the LDO requires additional power and area. In [2], a calibration mechanism is used to close and open the loop on an alternating basis, increasing the settling time. In [3], an extra foreground frequency calibration is needed to ensure the PLL is at the correct sub-band and supply voltage - it may be susceptible to process, voltage and temperature (PVT) variations. The approach in [4] requires a low-frequency triangular signal generator, additional current sources, and active devices, increasing power and noise. In [5], two constant-gm bias circuits are used to cancel supply noise. However, it is sensitive to PVT variations, which may degrade the supply-noise suppression.