A 90μW 12MHz Relaxation Oscillator with a -162dB FOM

Both ring oscillators and relaxation oscillators are subsets of RC oscillators featuring large tuning ranges and small areas. Figure 19.5.1 shows a typical relaxation oscillator with a capacitor and two switched current sources. Such relaxation oscillators have two advantages with respect to ring oscillators: 1) they have a constant frequency tuning gain; and 2) their phase can be read out continuously due to their triangular (or sawtooth) waveform. A major disadvantage of practical relaxation oscillators is their poor phase-noise compared to ring oscillators [1,2,4].