Design of a 3-bit digital control oscillator (DCO) using IMOS varactor tuning

Abstract

This paper presents a three-bit digital controlled oscillator (DCO) with three-stage ring topology. The proposed DCO circuit has been realized in TSMC 0.18-μm CMOS technology. DCO circuit has been designed with NAND gate based inverter delay cell for low power consumption and a digitally controlled load element is added at the output node of inverter to control the tuning range of proposed DCO circuit. Load element of DCO circuit has been implemented using three IMOS varactors to provide a better frequency tuning capability at low supply voltage (Vdd). The digital control bit varies the driving current across the load element that causes to change the oscillation frequency of proposed DCO. A tunable range of frequency from 0.818 to 0.872 GHz has been obtained with different combination of digital control bit with Vdd of 1.8 V. Effect of Vdd variations has also been observed. For control bits CBA = ‘000’ the proposed DCO operates from 0.711 to 1.278 GHz with Vdd variations from 1.6 to 3 V. Further, at control bits CBA = ‘111’, DCO oscillates form 0.755 to 1.296 GHz with Vdd variation. The proposed DCO shows a phase noise of − 101.24 dBc/Hz at an offset of 1 MHz from carrier frequency with 0.248 mW power consumption at 0.872 GHz operating frequency. Figure of merit for the proposed DCO circuit is 166.10 dBc/Hz.