Tuning Range Extension of a Transformer-Based Oscillator Through Common-Mode Colpitts Resonance

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In this paper, we propose a method to broaden a tuning range of a CMOS LC-tank oscillator without sacrificing its area. The extra tuning range is achieved by forcing a strongly coupled transformer-based tank into a common-mode resonance at a much higher frequency than in its main differential-mode oscillation. The oscillator employs separate active circuits to excite each mode but it shares the same tank, which largely dominates the core area but is on par with similar single-core designs. The tank is forced in common-mode oscillation by two injection locked Colpitts oscillators at the transformer's primary winding, while a two-port structure provides differential-mode oscillation. An analysis is also presented to compare the phase noise performance of the dual-core oscillator in common-mode and differential-mode excitations. A prototype implemented in digital 40-nm CMOS verifies the dual-mode oscillation and occupies only 0.12 mm2 and measures 56% tuning range.

Original languageEnglish
Article number7876717
Pages (from-to)836-846
Number of pages11
JournalIEEE Transactions on Circuits and Systems Part 1: Regular Papers
Issue number4
Publication statusPublished - 2017


  • Colpitts oscillator
  • Common-mode oscillation
  • inductor-capacitor (LC) tank
  • transformer
  • wide tuning range


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