LC VCO optimization & design

Donhee Ham (donhee@caltech.edu)

Integrated LC voltage controlled oscillators (VCO) are essential functional blocks in modern radio frequency communication systems and areused as local oscillators to up- and down-convert signals. Due to the ever-increasing demand for bandwidth, very stringent requirements are placed on the spectral purity of local oscillators. Efforts to improve the phase noise performance of integrated LC VCOs have resulted in a large number of realizations. Despite these endeavors, design and optimization of integrated LC VCOs still pose many challenges to circuit designers as simultaneous optimization of multiple variables is required.

A computer-aided optimization technique using geometric programming has been recently used to find the optimum design for certain LC oscillator topologies efficiently. Despite its efficiency, it provides limited physical insight into choosing the optimum design, as it completely relies on the computer to perform the optimization. Therefore, even in the presence of such CAD tools, firm understanding of the underlying trade-offs among the design parameters is essential to enhance circuit innovations and increase design productivity. This is specially important when the number of design parameters is large, as any optimization tool unjustifiably exploits the limitations of the models used.

My research on LC VCO design addresses this issue. Starting from the underlying themodynamic principles governing the noise process in LC oscillators, we came to the conclusion that inductance selection process plays a centralrole in oscillator noise optimization. An investigation of phase noise properties leads to a design strategy based on an inductance selection scheme, providing a basis fora detailed optimization methodology. This optimization process entails an intuitive graphical method to visualize the design constraints such as tank amplitude, frequency tuning range and start-up condition, allowing minimization of phase noise while satisfying all design constraints.

Experimental results and comparison with other reported LC oscillators prove the adequacy of our design methodology.