Low-phase-noise, high switching speed digitally controlled ring oscillator in 0.18μm complementary metal oxide semiconductor

In this paper a 14-bit digitally controlled oscillator (DCO) with operating frequency up to 4.2 GHz in 0.18 μm complementary metal oxide semiconductor (CMOS) technology is presented. To improve the phase noise, digital control is used that avoids continuous conduction of transistors and controlling transistors operate either in triode or cutoff region. The circuit uses multiple-loop feed forward architecture to increase the switching speed. The DCO has a wide frequency tuning range from 490 MHz to 4.2 GHz. The measured phase noise of the DCO is μ121.2 dBc/Hz at a 4-MHz offset from a 3.86-GHz center frequency and power consumption of 48 mW. Due to the phase noise reduction and high switching speed, the DCO has a μ164.1 dBc/Hz figure of merit (FOM) that is an improvement of μ4 dBc/Hz over the previously published results in the same technology. The chip area is 300 × 300 μm2.