Spectral Analysis of Noise in Circuits and Systems for Wireless Communications: New Thoughts on an Old Subject

Aleksandar Tasic
Qualcomm Incorporated

Tuesday, March 31, 2009 at 4:00pm
Engr IV Room 57-124

Abstract
Interpretation of phase-noise generating mechanism in oscillators relies on approximate and numerical calculations for analysis, and simulation tools for synthesis. The phase-noise models obtained are often only indicative, failing to describe the oscillator phase noise using circuit parameters. An intuitive, yet simple, phase-noise model for LC-oscillators is described in this presentation using spectral noise analysis. The model is amenable for design as it describes the noise performance of LC-oscillators qualitatively and quantitatively using electrical parameters. Noise contributions to phase noise of the LC-tank, transconductor, and bias circuitry are determined for bipolar and CMOS LC-oscillators. Their phasenoise performance is further compared, and fundamental limitations of both oscillator topologies identified. Resonant-inductive degeneration of bias current source for a manifold improvement of phase noise in LC-oscillators is then introduced. Two test oscillator designs are described confirming the validity of the phase-noise reduction method resulting in a 6dB phase-noise improvement. Low supply voltages of downscaled silicon technologies prohibit stacking of transistors in oscillators and limit the voltage swing of the oscillation signal. To account for the soft switching of the oscillator transconductor due to a smaller oscillation signal, the existing oscillator circuits and their phase-noise models need to be modified, as discussed in this presentation. The presentation is concluded with system noise-figure and phase-noise analysis of various receiver topologies aided by spectral interpretation. Receiver signal-to-noise ratio and noise figure are described as functions of phase-noise of oscillators and noise figure of low-noise amplifiers and mixers.