A 12MHz Switched-Capacitor Relaxation Oscillator with a Nearly Minimal FoM of -161dBc/Hz

In this work the phase noise performance of relaxation oscillators has been analyzed resulting in simple though precise phase noise expressions. These expressions have lead to a new relaxation oscillator topology, which exploits a noise filtering technique implemented with a switched-capacitor circuit to minimize phase noise. Measurements on a 65nm CMOS design show a sawtooth waveform, a frequency tuning range between 1 and 12MHz and a rather constant frequency tuning gain. At 12MHz oscillation frequency it consumes 90μW while the phase noise is -109dBc/Hz at 100KHz offset frequency. By minimizing and balancing noise contributions of charge and discharge mechanisms, a nearly minimal FoM of -161dBc/Hz has been achieved, which is a 6dB improvement over state-of-the-art.