A 2.4-GHz ring-oscillator-based CMOS frequency synthesizer with a fractional divider dual-PLL architecture

A 2.4-GHz frequency synthesizer was designed that uses a fractional divider to drive a dual-phase-locked-loop (PLL) structure, with both PLLs using only on-chip ring oscillators. The first-stage narrow-band PLL acts as a spur filter while the second-stage wide-band PLL suppresses VCO phase noise so that simultaneous suppression of phase noise and spur is achieved. A new low-power, low-noise, low-frequency ring oscillator is designed for this narrow-band PLL. The chip was designed in 0.35-μm CMOS technology and achieves a phase noise of -97 dBc/Hz at 1-MHz offset and spurs of -55 dBc. The chip´s output frequency varies from 2.4 to 2.5 GHz; the chip consumes 15 mA from a 3.3-V supply and occupies 3.7 mm°.