Physical processes of phase noise in differential LC oscillators

  • Rael J
  • Abidi A
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There is an unprecedented interest among circuit designers today
to obtain insight into the mechanisms of phase noise in LC oscillators.
For only with this insight is it possible to optimize oscillator
circuits using low-quality integrated resonators to comply with the
exacting phase noise specifications of modern wireless systems. In this
paper we concentrate on an understanding of the popular differential LC
oscillator. We introduce simple models to capture the nonlinear
processes that convert voltage or current thermal noise in resistors or
transistors into phase noise in the oscillator. The analysis does not
require hypothetical elements, such as limiters or amplitude control
loops, to fully explain phase noise. A simple expression at the end
accurately specifies thermally induced phase noise, and lends substance
to Leeson's original hypothesis. Next, the upconversion of flicker noise
into phase noise is traced to mechanisms first identified in the 1930's,
but apparently since forgotten. Unlike thermally induced phase noise,
which appears as phase modulation sidebands, flicker noise is shown to
upconvert by bias-dependent frequency modulation. The results are
validated against SpectreRF simulations and measurements on two
differential CMOS oscillators tuned by resonators with very different

Author-supplied keywords

  • 1f noise
  • Frequency modulation
  • Phase noise
  • Resonant frequency
  • Thermal noise

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  • J. J. Rael

  • A. A. Abidi

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