A 0.022mm2 970µW dual-loop injection-locked PLL with −243dB FOM using synthesizable all-digital PVT calibration circuits

Abstract

For modern SoC systems, stringent requirements on on-chip clock generators include low area, low power consumption, environmental insensitivity, and the lowest possible jitter performance. Multiplying Delay-Locked Loop (MDLL) [1-2], subharmonically injection-locked techniques [3], and sub-sampling techniques [4-5] can significantly improve the random jitter characteristics of a clock generator. However, in order to guarantee their correct operation and optimal performance over process-voltage-temperature (PVT) variations, each method requires additional calibration circuits, which impose difficult-to-meet timing constraints. In the case of an injection-locked PLL (IL-PLL), a free-running frequency calibration is required. However, the output of an injection-locked oscillator is always fixed at the desired frequency, so a shift in the free-running frequency (e.g. caused by temperature and voltage variations) cannot be simply compensated for by using a frequency-locked loop (FLL). Therefore, we propose the use of a dual-loop topology with one free-running voltage-controlled oscillator (VCO) as a replica VCO placed inside a FLL for tracking temperature and voltage drift. The other VCO (the main VCO) is injection locked for producing a low-jitter clock, while the free-running frequency shift can be compensated for by the replica loop. The method provides robust output over temperature and voltage variations.