Low phase noise CMOS voltage-controlled oscillators

Design considerations and performance comparisons for several low phase noise CMOS voltage-controlled oscillator (VCO) topologies are presented including the Hartley, quadrature Colpitts, Clapp, and tuned-input tuned-output configurations. An indirect approach for high-frequency signal generation using a VCO coupled with a 2X passive frequency multiplier is also described. Several of the structures are attractive alternatives to the conventional LC tank VCO.

I. Introduction

Achieving excellent noise performance is one of the foremost requirements for a CMOS receiver front end. In such a system, a voltage-controlled oscillator (VCO) is employed in a frequency synthesizer to generate the local oscillator (LO) signal that drives the receive mixer. The phase noise of the VCO directly affects the spectral purity of the received signal in many ways, especially when used in a low loop-bandwidth synthesizer. The cross-coupled LC oscillator is one of the most widely used circuit configurations in modern CMOS implementations. However, the need to continuously reduce the phase noise of CMOS VCOs has led to active research of several other basic oscillator topologies.