A study of phase noise in colpitts and LC-tank CMOS oscillators

Pietro Andreani, Xiaoyan Wang, Luca Vandi, A. Fard

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This paper presents a study of phase noise in CMOS Colpitts and LC-tank oscillators. Closed-form symbolic formulas for the 1/f(2) phase-noise region are derived for both the Colpitts oscillator (either single-ended or differential) and the LC-tank oscillator, yielding highly accurate results under very general assumptions. A comparison between the differential Colpitts and the LC-tank oscillator is also carried out, which shows that the latter is capable of a 2-dB lower phase-noise figure-of-merit (FoM) when simplified oscillator designs and ideal MOS models are adopted. Several prototypes of both Colpitts and LC-tank oscillators have been implemented in a 0.35-mu m CMOS process. The best performance of the LC-tank oscillators shows a phase noise of -142 dBc/Hz at 3-MHz offset frequency from a 2.9-GHz carrier with a 16-mW power consumption, resulting in an excellent FoM of similar to 189 dBc/Hz. For the same oscillation frequency, the FoM displayed by the differential Colpitts oscillators is similar to 5 dB lower.
Original languageEnglish
JournalI E E E Journal of Solid State Circuits
Issue number5
Pages (from-to)1107-1118
Publication statusPublished - 2005

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Copyright: 2005 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE


  • LC-tank
  • Colpitts
  • CMOS
  • phase noise
  • oscillators


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