2154 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 51, NO. 11, NOVEMBER 2003 A First Multigigahertz Digitally Controlled Oscillator for Wireless Applications Robert Bogdan Staszewski, Member, IEEE, Chih-Ming Hung, Member, IEEE, Dirk Leipold, and Poras T. Balsara, Senior Member, IEEE Abstract—A novel digitally controlled oscillator (DCO) ar- chitecture for multigigahertz wireless RF applications, such as short-range wireless connectivity or cellular phones, is proposed and demonstrated. It deliberately avoids any use of an analog tuning voltage control line. Fine frequency resolution is achieved through high-speed dithering, yet the resulting spurious tones are very low. This enables to employ fully digital frequency synthe- sizers in the most advanced deep-submicrometer digital CMOS processes, which allow almost no analog extensions. It promotes cost-effective integration with the digital back-end onto a single silicon die. The demonstrator test chip has been fabricated in a digital 0.13- m CMOS process together with a digital signal processor to investigate noise coupling. The 2.4-GHz DCO core consumes 2.3 mA from a 1.5-V supply and has a very large tuning range of 500 MHz. The phase noise is 112 dBc/Hz at 500-kHz offset. The presented ideas have been incorporated in a commer- cial BLUETOOTH transceiver. Index Terms—CMOS digital integrated circuits (ICs), digital control, digitally controlled oscillator (DCO), MOS varactor, voltage-controlled oscillators (VCOs). I. INTRODUCTION T RADITIONAL designs of commercial frequency synthe- sizers for multigigahertz mobile RF wireless applications have almost exclusively employed the use of a charge-pump phase-locked loop (PLL), which acts as a local oscillator (LO) for both a transmitter and receiver. Unfortunately, the design flow and circuits techniques required are quite analog inten- sive and utilize process technologies that are incompatible with a digital baseband, which, today, is built in a low-voltage deep- submicrometer digital CMOS process with almost no analog ex- tensions and very limited voltage headroom. Furthermore, the charge-pump-based PLL suffers from reference spurs due to its specific method of correlative phase comparison [1]. The aggressive cost and power reduction of a mobile wireless solution can only be realistically achieved by the highest level of integration, and this favors digitally intensive approach. A dig- itally controlled oscillator (DCO) that deliberately avoids any analog tuning voltage controls is presented in this paper. This allows for its loop control circuitry, including loop filter, to be implemented in a fully digital manner. Thus far, there have not been any reports in the literature (except recently by the authors in [2]) on the fully digital con- Manuscript received April 16, 2003; revised June 15, 2003. R. B. Staszewski, C.-M. Hung, and D. Leipold are with Texas Instruments Incorporated, Dallas, TX 75243 USA. P. T. Balsara is with the Department of Electrical Engineering, University of Texas at Dallas, Richardson, TX 75083 USA. Digital Object Identifier 10.1109/TMTT.2003.818579 Fig. 1. Idealized capacitance versus voltage curves of a MOS varactor for both a traditional and a deep-submicrometer CMOS process. trol of oscillators for RF applications. Lack of the fully dig- ital control is a severe impediment for the total integration in a deep-submicrometer CMOS process. There have been several disclosures on ring-oscillator-based DCOs for clock recovery and clock generation applications [3], [4]. However, the fre- quency resolution is low and the spurious tone level is high for these DCOs, which seem to become an effective deterrent against digital RF synthesizers for wireless communications. The proposed combination of various circuit and architectural techniques has brought to fruition a fully digital solution that has a fine frequency resolution with low spurious content and low phase noise. This paper is organized as follows. Section II discusses choices behind the selected varactor and its proposed mode of operation. Section III proposes a novel fully digital manner of an LC-tank oscillator control. LC-tank details are presented in Section IV. The oscillator core is described in Section V. Section VI derives an equation for the DCO switching spurs and shows through some scenario examples that the resulting spurs could be made vanishingly small. An alternative view of the DCO oscillator as a digital-to-frequency converter is given in Section VII. The implementation and measured results are presented in SectionsVIII and IX. II. VARACTOR IN A DEEP-SUBMICROMETER CMOS PROCESS Frequency tuning of a low-voltage deep-submicrometer CMOS oscillator is quite a challenging task due to its highly nonlinear frequency-versus-voltage characteristics and low- voltage headroom. Fig. 1 shows normalized representative curves of a MOS varactor capacitance versus control voltage 0018-9480/03$17.00 © 2003 IEEE Authorized licensed use limited to: Seoul National University. Downloaded on October 27, 2009 at 04:48 from IEEE Xplore. Restrictions apply.