Digital sensor for IR radiation

Abstract

Digital signal processing and microprocessors favour digital sensors rather than analog sensors with subsequent A/D conversion. To extend digital sensor techniques to the far infrared, we used a special small quartz crystal. Its resonance frequency depends strongly on the temperature. Using IR-absorbing electrodes, the resonance frequency depends linearly on the radiation intensity. To eliminate thermal drift, a chopper technique and a digital counter are used to provide a direct digital measurement without any intermediate analog electronic signals. Current sensors show frequency variations of 3 Hz/mK, which can be increased to 14 Hz/mK with a fifth overtone oscillator. Using a down converter with a second stable quartz oscillator and a reciprocal counting technique can eliminate the 1/ T resolution limit of conventional counters. Preliminary experiments with black body radiation of various temperatures show sensitivities of several MHz/W and a detection limit near 10 −7 W.