Thermal coupling and effect of subharmonic synchronization in a system of two VO2 based oscillators

Abstract

We explore a prototype of an oscillatory neural network (ONN) based on vanadium dioxide switching devices. The model system under study represents two oscillators based on thermally coupled VO2 switches. Numerical simulation shows that the effective action radius RTC of coupling depends both on the total energy released during switching and on the average power. It is experimentally and numerically proved that the temperature change ΔT commences almost synchronously with the released power peak and T-coupling reveals itself up to a frequency of about 10 kHz. For the studied switching structure configuration, the RTC value varies over a wide range from 4 to 45 μm, depending on the external circuit capacitance C and resistance Ri, but the variation of Ri is more promising from the practical viewpoint. In the case of a "weak" coupling, synchronization is accompanied by attraction effect and decrease of the main spectra harmonics width. In the case of a "strong" coupling, the number of effects increases, synchronization can occur on subharmonics resulting in multilevel stable synchronization of two oscillators. An advanced algorithm for synchronization efficiency and subharmonic ratio calculation is proposed. It is shown that of the two oscillators the leading one is that with a higher main frequency, and, in addition, the frequency stabilization effect is observed. Also, in the case of a strong thermal coupling, the limit of the supply current parameters, for which the oscillations exist, expands by ~ 10 %. The obtained results have a universal character and open up a new kind of coupling in ONNs, namely, T-coupling, which allows for easy transition from 2D to 3D integration. The effect of subharmonic synchronization hold promise for application in classification and pattern recognition.

A Model of an Oscillatory Neural Network with Multilevel Neurons for Pattern Recognition and Computing

“…To denote high-order synchronization value for N interacting oscillators in [30] we worked out the ratio of integers k1:k2:k3:..kN, where kN is harmonic order of N-th oscillator at the common frequency of the network synchronization Fs. In our case when we define synchronization order only between two oscillators (№1 и №2-11), it is possible to use the concept of subharmonics ratio as in [12], that is defined as a simple fraction SHRi,j=ki:kj ~ ki/kj , where i and j are the oscillators numbers between which the synchronization order is measured. It is obvious that to define the synchronization order for the first oscillator SHR1,1 in relation to itself is not reasonable because SHR1,1=1.…”

“…Also pulse type ONNs with multifrequent periodic oscillation spectrum feature a specific mode of multiple frequency synchronization or, in other words, high order synchronization effect [11]. We have recently demonstrated this effect experimentally by the example of thermallycoupled VO2-oscillators [12]. In relaxation oscillators with vanadium dioxide-based film elements electric self-oscillations are activated by the effect of electric switching governed by metalinsulator transition (MIT) [13]- [16].…”

A Method for Evaluating Chimeric Synchronization of Coupled Oscillators and Its Application for Creating a Neural Network Information Converter

“…Such ONN are interesting due to simplicity of hardware implementation, as developed micro-and nano-electronic autogenerators ensure network compactness and energy efficiency. In addition, a pulsed-type ONN, where the periodic oscillation spectrum has a multi-frequency character, has a special mode of harmonics synchronization, or in other words, has a high order synchronization effect [24][25][26]. This effect has been demonstrated experimentally using the example of thermally coupled VO 2 oscillators [24].…”

Capacitorless model of a VO2 oscillator

“…Figure 4a presents the oscillograms of the current Isw and voltage Vsw on the switch in the oscillator model with a capacitor (Figure 1b) at IDD = 0.6 mA and C = 10 pF. Voltage oscillations with a period of T ~ 120 ns occur in the Vsw range from Vh '= 0.5 V to Vth ' = 5 V. The dynamic threshold characteristics (Vh ' , Vth ' ) differ from the static characteristics (Vh, Vth), and it is usually observed at high frequencies [13]. The charging of the capacitor C is accompanied by an increase in Vsw from Vh' to Vth ' , and the discharge of the capacitor is characterized by a sharp drop in Vsw from Vth ' to Vh' and an increase in current Isw, observed in Figure 4a.…”