Badanie właściwości oscylatora pierścieniowego w temperaturze 77 K

W artykule przedstawiono wyniki eksperymentów, w których testowano działanie oscylatora pierścieniowego zaimplementowanego w układach reprogramowalnych. Analizowano właściwości opóźniające inwerterów zaprogramowanych w strukturze CPLD układów XC2C32 (Xilinx). W temperaturze otoczenia (300 K) i w temperaturze ciekłego azotu (77 K), badano zdolność do generacji drgań, stałość częstotliwości oscylatora (na podstawie pomiarów średniookresowych), wpływ zmian napięcia zasilania na częstotliwość oscylacji.

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In this paper the results of experiments with a ring oscillator implemented in programmable devices (XC2C32 Xilinx) are presented. The examined devices were immersed in a Dewar flask (Fig. 1) with liquid nitrogen. It was found out that the ring oscillator (composed of 11 gates) (Fig. 2) still worked properly in such low temperature. According to the theory of silicon semiconductors, the activity of carriers increases in low temperatures, so there was expected decrease in the propagation delay for every gate and increase in the oscillation frequency. The output frequency was measured and the average propagation time for inverters was calculated. The results at 77 K (temperature of liquid nitrogen) were compared with those at 300 K (room temperature) (Tab. 1). The output frequency characteristics versus the supply voltage for the examined devices were measured and drawn (Figs. 3 and 4). The quadric polynominal functions which fit these non-linear characteristics were proposed. The relative change of the oscillation frequency versus the supply voltage is shown in Fig. 5. The frequency sensitivity depends both on supply voltage and temperature. The relative sensitivity (normalized) in relation to the voltage at 300 K and 77 K is presented in Fig. 6. Based on the results from 24-hour measurements (86400 samples were collected) the frequency stability was determined. The average value and standard deviation value were calculated (Tab. 2) but first and foremost there was calculated and plotted the Allan deviation (Fig. 7).