ABSTRACT
In this chapter, the authors discuss and analyze a class of operation of an RF oscillator that further minimizes its phase noise. They specifically address the ultra-low phase noise design space while maintaining high power efficiency. The authors describe a soft-clipping class-F2 oscillator topology based on enforcing a clipped voltage waveform around the LC tank by increasing the second-harmonic of the fundamental oscillation voltage through an additional impedance peak. Furthermore, the special impulse sensitivity function of the soft clipping waveform reduces significantly the circuit-to-phase-noise conversion. The additional resonant frequency is realized by exploiting a different transformer behavior in common-mode and differential mode excitations. In addition, the tank input impedance is also scaled down without sacrificing its Q-factor. Consequently, this structure is able to push the phase noise much lower than practically possible with the traditional LC oscillators while satisfying long-term reliability requirements.
