Comparison of parameters of ring and LC-tank digitally controlled oscillators in 0.13 μm CMOS

Abstract

Integrated circuits (IC) of two digitally controlled oscillators (DCO) are presented in this article – ring and LC-tank oscillator. Main parameters of these DCOs are measured and compared, applications of such DCOs are discussed. Ring DCO is made of 48 sections of 3-stage ring oscillators. Base component of ring oscillator is tristate inverter, which is accessible in modern CMOS technologies, therefore, such oscillator can be implemented using digital IC design tools. Main components of LC-tank DCO are inductor, switched-capacitor bank, and a pair of cross-coupled NMOS transistors. Frequency of ring DCO is controlled by enabling or disabling its separate sections, while frequency of LC DCO is controlled by switching capacitors of LC tank. Although LC DCO is controlled digitally, its core is analog and it can be implemented only using custom design tools. Both DCOs are designed and their prototypes are manufactured in mature and low-cost 0.13 μm RF CMOS technology. Ring DCO’s frequency tuning range is 370–873 MHz; phase noise at maximum operating frequency at 1 MHz offset from carrier is –109.19 dBc/Hz; power consumption is 3.6 mW; used area of silicon is 0.0065 mm2. LC-tank DCO’s frequency tuning range is 9.26–10.58 GHz; phase noise at maximum operating frequency at 1 MHz offset from carrier is –106.46 dBc/Hz; power consumption is 18 mW; used area of silicon is 0.06 mm2. Operating frequency of LC-tank DCO is much higher, however Figures of Merit of ring and LC-tank DCOs are comparable and respectively equal to –140.61 dBc/Hz and –136.88 dBc/Hz. It shows, that both DCOs can be used in modern systems for appropriate applications. Main applications of LC-tank DCO are high-speed systems, where high frequency is needed, while in low-cost, low-power systems ring DCO is more suitable.