| IEEE ISSCC 2008 |
A 0.5-to-480MHz Self-Referenced CMOS Clock Generator with 90ppm Total Frequency Error and Spread-Spectrum Capability |
| Abstract: |
The quartz crystal (XTAL) oscillator is one of the last components in electronic systems that has yet to be integrated. The advent of RF CMOS circuits and associated advances in CMOS process technology have enabled the development of low-noise integrated LC oscillators (LCOs), which are suitable for replacing XOs in USB applications. This work demonstrates a self-referenced CMOS LCO, or CMOS harmonic oscillator (CHO), that exhibits 90ppm total frequency error over process, bias and temperature, thus making it suitable for replacing XOs in many applications. Additionally, the clock generator can be configured to produce a number of different output frequencies, and includes a direct modulation technique enabling SSCG. |
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| IEEE JSSC 2007 |
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 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 +85oC. Measured period and cycle- to-cycle jitter are 6.78psrms and 8.96psrms, respectively. Fabricated in a 0.35mm 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. |
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