In this paper, an analytic approach for the estimation of the phase and amplitude error in series coupled LC quadrature oscillator (SC-QO) is proposed. The analysis results show that imbalances in source voltage of coupling transistor because of mismatches between LC tanks are the main source of the phase and amplitude error in this oscillator. For compensation of the phase and amplitude error, a phase and amplitude-tunable series coupled quadrature oscillator is designed in this paper. A phase shift generation circuit, designed using an added coupling transistor, can control the coupling transistor source voltage. The phase and amplitude error can simply be controlled and removed by tuning the phase shifter, while this correction does not have undesirable impact on phase noise. In fact, the proposed SC-QO generates a phase shift in the output current, which reduces the resonator phase shift (RPS) and improves phase noise. The phase and amplitude tunable SC-QO is able to correct the phase error up to 12°, while amplitude imbalances are reduced as well. To evaluate the proposed analysis, a 4.5-GHz CMOS SC-QO is simulated using the practical 0.18-μm TSMC CMOS technology with a current consumption of 2 mA at 1.8-V supply voltage.
