Fig. 3: Illustration of ONN computing paradigm and its circuit implementations.

a Illustration of a frequency-based ONN. The input consists of frequency-dependent signals fed into the ONN. The latter can have oscillators locking to a common frequency and providing the computational result. Labels of locked and unlocked are shown next to the oscillators. b Schematic of a phase-based ONN. The oscillators have a uniform frequency and evolve in the phase domain, minimizing a kind of Hopfield energy. c Two coupled oscillators implemented with various technologies. Ring oscillators can be coupled by back-to-back inverters⁶⁸ or using transmission gates⁸⁷. Relaxation oscillators consist of a hysteresis device that charges/discharges a load capacitor, producing analog oscillations. The hysteresis component can be implemented by a Schmitt trigger^86,201 or beyond-CMOS devices that have a negative differential resistance region in their I–V characteristic. A partial list of potential devices includes VO₂^62,63,93,260, TaO_x and TiO_x oxide⁸⁹, NbO_x memristors⁹⁰, PrMnO₃ RRAM⁹¹, or ferroelectric transistors⁹².