A monolithic and self-referenced RF LC clock generator compliant with USB 2.0

A monolithic and self-referenced radio frequency (RF) LC clock generator that is compliant with USB 2.0 is demonstrated 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 frequency 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 temperature variation from -10 to +85 °C. Measured period and cycle-to-cycle jitter are 6.78 psrms and 8.96 psrms, respectively. Fabricated in a 0.35 μm CMOS technology, the clock generator occupies 0.22 mm2 and draws 9.5 mA from a 3.3-V supply, which is derived from the 5-V USB power supply.