Razavi´s oscillator theory interpreted in RF circuit port impedance domain to harmonize with Leeson´s sideband noise spectrum model

Frequency spectrum purity or phase noise is ever of concern to oscillator engineers. The spectrum model proposed by Leeson predicts noise power density to be observed at frequency offset around the oscillation, provided that the device noise figure and the factor labelled “Q” are given. To develop low-noise oscillators, circuit designers need to know how to estimate the Q from their circuits. This paper traces Q factor formulas derived for oscillators in block-diagram and circuit-scheme models. We find how the Q factor depends upon the oscillator´s topology. Typical examples of positive-feedback, negative-resistance, and transistor oscillators are shown. The theory contributes to helping us create new schemes exhibiting high spectrum purity.