Design and Analysis on Bidirectionally and Passively Coupled QVCO With Nonlinear Coupler

This paper studies quadrature voltage-controlled oscillators (QVCOs) employing bidirectional current injection from the two adjacent terminals and using a passive nonlinear coupler (BPN-QVCO). For such a QVCO to achieve quadrature locking while being robust to components mismatch, the odd-order nonlinearity of the coupling cell is essential and its significance in the performance merits is introduced. To verify the provided theory on BPN-QVCOs with resistive couplers, a modified diode is proposed as the coupling cell with doubled third-order nonlinearity for improved phase error without introducing additional capacitive loading. The proposed prototype and a standard 26-GHz BPN-QVCO using conventional diode couplers are fabricated in 65-nm CMOS. The phase error due to the LC-tank mismatch is demonstrated to be half with the proposed coupling cell, verifying the theory. The proposed theory also applies to BPN-QVCOs using nonlinear capacitors as coupling cells.