Low 1/f 3 Phase Noise Quadrature LC VCOs

Series-coupled quadrature LC voltage-controlled oscillators (SQVCOs) perform robustly over a wide tuning range, but have a higher 1/f(3) phase noise than their single-phase counterparts. Switching transistors inject noise into the tank only once per cycle leading to an asymmetric impulse sensitivity function (ISF) and large flicker noise upconversion. Circuit topologies with additional capacitive paths from the drains of the switching transistors are proposed to make the switching transistors inject noise twice in a cycle and reduce the average ISF to nearly zero. The proposed SQVCOs have a phase noise figure of merit (FoM) close to that of a single-phase VCO for a given tank. They also eliminate mode ambiguity usually present in the coupled VCOs. The 2.4 GHz prototypes of two of the proposed SQVCOs in a 0.13 mu m CMOS process have 1/f(3) phase noise corners of 400 and 150 kHz compared to 3 MHz in the conventional SQVCO. At 1 MHz/3 MHz offset, the proposed SQVCOs have measured phase noise (FoM) of -133.5/-145.1 dBc/Hz (188.2/190.2 dB) and -130/-140.2 dBc/Hz (192.5/193.2 dB) whereas the conventional SQVCO has -125.5/-137.6 dBc/Hz (180.3/183 dB). The average measured phase error over five chips is 0.9 degrees.