Effect of device mismatch on the phase and amplitude accuracy in LC quadrature VCOs

An overview of cross-coupled LC tank quadrature VCOs is presented. Then, the effect of mismatch on the amplitude and quadrature phase accuracy is discussed. In particular, we focus on the mismatch between the negative resistance (R_neg) and quadrature resistance (R _quad) of the two VCOs. By considering a first order linear model of the QVCO, it is shown that a mismatch in R_neg generates an imbalance in the quadrature phase but preserves the amplitude accuracy, whereas a mismatch in R_quad leads to both a relative amplitude error and phase imbalance. Furthermore, the mismatch leads to an effective coupling transconductance G_Mceff that in turn causes a deviation in the steady state oscillation frequency. The developed analysis is verified using SpectreRF simulation of a parallel QVCO topology (PQVCO)