On the amplitude and phase errors of quadrature LC-tank CMOS oscillators

An analytic approach for the estimation of the phase and amplitude imbalances caused by component mismatches and parasitic magnetic fields in two popular quadrature LC oscillators is presented. Very simple and closed-form equations are derived, proving that, although the two topologies share the same small signal circuit, they display very different sensitivities to the mentioned sources of imbalance. Moreover, it is shown that parasitic inductors coupling, overlooked up to date, plays a key role ultimately limiting the achievable phase accuracy. The theoretical results are verified through extensive simulations and measurements on a 1.7-GHz quadrature oscillator and frequency up-converter implemented in a 0.18-μm CMOS process.