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A Jitter-Cleaning Fractional-N Frequency Synthesizer with 10 Hz-40 kHz Digitally Programmable Loop Bandwidth


This dissertation contains three parts. In the first part, the analysis and circuits of a jittercleaning fractional-N frequency synthesizer is presented. In the second part, a low phase noise and low I/Q mismatch quadrature VCO is presented. In the third part, a low phase noise digital PLL is presented.

For the first part, the design utilizes a dual-loop architecture, which is suitable for integration in an SoC environment. The primary loop is a digital PLL with a second-order noise shaping phase-error ADC. The secondary loop is a fractional-N PLL implementing the digitally controlled oscillator inside the primary loop, and it locks to an external clean reference clock to reduce the phase noise and to improve the frequency stability of the on-chip oscillator. For the second part, a tail-tank coupling technique that combines two complementary differential LC-VCOs to form a quadrature LC-VCO is presented. This technique reduces phase noise by providing additional energy storages for noise redistribution and by canceling out most of the noise injected by transistors when they operate in the triode region. The resulting noise factor is close to the theoretical minimum value.

For the third part, a 2.8 to 3.2 GHz fractional-N digital PLL is presented. A divider with two-stage retiming improves linearity to reduce fractional spurs without increasing the in-band noise floor. An ADC is employed to boost TDC resolution by five times to achieve 2 ps effective resolution. A dither-less DCO with an inductively coupled fine-tune varactor bank improves tuning step-size to 20 kHz. With a 52 MHz reference clock and a loop-bandwidth of 950 kHz, this prototype achieves 230 fs rms jitter integrated from 1 kHz to 40 MHz offset while drawing 17 mW from a 1.8V supply. A FOM of -240.4 dB is achieved.

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