A Monolithic and Self-Referenced RF LC Clock Generator Compliant With USB 2.0

Abstract

A monolithic and self-referenced radio frequency (RF) LC clock generator that is compliant with USB 2.0 is demon- strated in a system-on-chip (SoC). This work presents the first successful approach to replacing an external crystal (XTAL), the crystal oscillator (XO) and the phase-locked loop for clock generation in an IC supporting USB 2.0 using a standard CMOS fabrication process. It is shown that the primary design challenges with the implemented approach involve maintaining high fre- quency accuracy and low jitter. Techniques for addressing both are shown. In particular, the presented architecture exploits the effects of frequency division and low far-from-carrier phase noise to achieve low jitter. From a 1.536 GHz temperature-compensated LC reference oscillator, coherent clock signals are derived at 96 MHz for the SoC logic and 12 MHz for an on-chip full-speed USB PHY. Though self-referenced, approximately 400 ppm total frequency accuracy is achieved over process variations, 10% variation in the USB power supply voltage and tem- perature variation from 10 to /4385 C. Measured period and cycle-to-cycle jitter are 6.78 ps /114/109/115and 8.96 ps /114/109/115, respectively. Fabricated in a 0.35 m CMOS technology, the clock generator occupies 0.22 mm/50and draws 9.5 mA from a 3.3-V supply, which is derived from the 5-V USB power supply.