In this paper, the effects that limit the performance of practical implementations of RC relaxation oscillators are investigated. The insights gained are used to suggest a topology for high-frequency quadrature relaxation oscillators with closer-to-optimal performance. The proposed oscillator uses a modified latch to improve the switching speed without increasing the power consumption. Moreover, the new topology avoids static current sources, maximizes the voltage swing and has an active coupling structure without static power consumption that reduces the circuit phase-noise. Experimental results show that the oscillator operates in relaxation mode at 2.4 GHz and achieves a FoM of $$\mathrm {-162\,dBc/Hz}$$ - 162 dBc / Hz , which is, as far as the authors know, the best FoM for relaxation oscillators operating in the GHz range.