Contents 1. Introduction
Integrated voltage-controlled oscillators (VCOs) are important building elements in the implementation of a single chip radio in today's communication systems. particular, RF oscillators employed in wireless transceivers must meet stringent phase noise requirement, typically mandating the use of passive LC tanks with a high quality factor . Local oscillators have the major source of phase noise witch is produced by their active elements, and pass through the circuit and affect the output spectrum. These effects can be decrease by choosing a proper topology of CMOS circuit, in other word, by changing the noise path from source nodes to output node. Cross-coupled oscillators have been preferred over other topologies due to their ease of implementation [4], relaxed start-up condition and differential operation. However, in cross-coupled oscillators, the noise generation by the active devices occurs when the oscillator is quite sensitive to perturbations, degrading the phase noise considerably. On the other hand, the Colpitts oscillator can potentially achieve lower phase noise. Despite these advantages, single-ended Colpitts oscillators are rarely used in today's integrated circuits due to their higher required gain for reliable start-up and single-ended nature that makes them more sensitive to parameter variations and common-mode noise sources, such as substrate and supply noise. Moreover, quadrature signals are typically required in many receiver and transmitter architectures, where the accuracy of such quadrature signals would determine the image rejection. A differential quadrature oscillator is thus preferred over a single-ended one.
