Intuitive Understanding of Flicker Noise Reduction via Narrowing of Conduction Angle in Voltage-Biased Oscillators

This brief aims to intuitively explain and numerically verify the observed phenomenon of flicker noise reduction in oscillators of reduced conduction angle (i.e., in class-C), which has been presented in literature but never properly explained. The flicker phase noise in a voltage-biased oscillator capable of operating in class-B and class-C is compared and numerically verified using a commercial simulation model of TSMC 28-nm CMOS. We illustrate how narrowing the conduction angle can suppress the 1/f noise up-conversion by decreasing 1/f noise exposure to the asymmetric rising and falling edges of oscillation waveform. The effects of implicit common-mode tank in the class-C operation is also discussed. We further clarify ambiguities among several simulation methods of impulse sensitivity function (ISF) based on periodic small-signal analysis [periodic AC or periodic transfer function (PXF)], which is a key tool in understanding the flicker noise up-conversion. A clearer ISF simulation method based on positive sidebands of PXF is proposed.