A Colpitts CMOS Quadrature VCO Using Direct Connection of Substrates for Coupling

A new low-phase noise low-power quadrature voltage-controlled oscillator (QVCO) using differential Colpitts oscillator is presented. The proposed QVCO is composed of two identical current-switching differential Colpitts VCOs in which the first core VCO is coupled to the second in an in-phase manner, and the second core VCO is coupled to the first in an anti-phase manner. To couple the two core VCOs, the substrates of the cross-connected transistors as well as the substrates of MOS varactors are used; alleviating the need for any extra elements for coupling, which could add noise and increase power dissipation. A linear (sinusoidal) analysis is presented that confirms that the proposed circuit generates quadrature waveforms. The proposed coupling technique can be generalized to N differential Colpitts VCOs for multiphase signals generation.