A current shaping technique to lower phase noise in LC oscillators

A novel LC-tank oscillator, where bias noise is decoupled and drain current is shaped in order to reduce phase noise, is presented. Active devices work in a class-C manner, the drain current is pulse shaped and peaks when the oscillator is insensitive to perturbations. Based on the ISF theory, this current shape can improve phase noise performance. Closed-form 1/f² phase noise equations are derived and compared with those of the classic LC-tank oscillator with the same bias and LC-tank. Simulations agree with conclusions from the theory and show 5.5 dB improvement in 1/f² phase noise at 1-MHz offset.