Spectrum folding and phase noise in LC tuned oscillators

We present a theory of the noise transfer in LC tuned oscillators accounting for the nonlinear operation of the transconductor. We show that the transconductor switching causes a folding of the wide-band noise such as the thermal noise of the spreading resistance of the bipolar transistors and the noise of the tail current generator. The effect is similar to what happens in sampled systems, however, for a careful evaluation of the oscillator phase noise, the correlations between the folded terms is of chief importance. We show how to account for the effect, we assess the impact on the oscillator noise performance and we give the guidelines for the circuit optimization.