A PVT Variation Dependencies of VCO in Frequency Locked Loop

This paper discusses the dependencies of PVT (process, voltage, temperature) variation in a frequency locked loop (FLL). The FLL has good frequency stability. However, the available supply voltage range and temperature range are limited in the actual design because of the PVT variation of the VCO. In particular, PVT variation becomes a problem when the output frequency of FLL is close to the upper and lower limits of the frequency range of VCO. In this paper, we propose a VCO based on an adjustable Schmitt trigger and design the FLL at a frequency close to the lower limit of the VCO. The VCO with a wide frequency range or a high PVT variation tolerance can realize the FLL with low PVT variation dependencies. The PVT variation in FLL is dominated by the PVT variation in the VCO according to the experimental results.