Tunable VO₂ relaxation oscillators for analog applications

The reversible insulator-to-metal phase transition of vanadium dioxide (VO₂) makes this material an ideal candidate to realize electrical oscillators with both tunable frequency and amplitude characteristics. Here we report results for VO₂ oscillators operating under voltage actuation. Large amplitude electrical oscillation waveforms with stable frequency were observed for the investigated devices. The frequency and amplitude of the waveforms were controlled by changing the values of the capacitor in the oscillator circuit, the supply voltage and by varying the temperature of the devices. An equivalent circuit model was developed to simulate the waveforms. A good agreement between simulations and experiments was verified for different oscillator circuit conditions. We also present a graphical phase portrait analysis of the differential equations governing the time-dependent external capacitor voltage in the oscillator circuit. A simple technique to analyze the operating conditions of these devices was developed based on this approach using the oscillator circuit parameters and the resistances of the VO₂ device in the insulating and metallic states.