This article presents an ultra-low-noise differential relaxation oscillator that achieves a phase noise figure of merits (FoMs) of 157.7 and 162.1 dBc/Hz, respectively, at 1- and 100-kHz frequency offsets. The oscillator is inherently robust against 1/f noise, while swing-boosting minimizes the phase noise arising out of thermal noise. Furthermore, an inverter-based differential comparator maximizes power efficiency, enabling FoMs close to the fundamental limits. Operating at 10.5 MHz and consuming 219.8 μ W from a 1.4-V supply, the oscillator achieves a period jitter of 9.86 psrms, equivalent to a relative jitter of 0.01%. The oscillator occupies an active area of 0.015 mm2 in a 0.18- μ m standard CMOS process.