An RF-Powered Wireless Temperature Sensor for Harsh Environment Monitoring With Non-Intermittent Operation

Abstract

This paper presents a fully integrated RF-powered temperature sensor with non-intermittent operation. The sensor is powered up wirelessly from a 915-MHz incident signal using a power-efficient RF energy harvester, uses a subthreshold ring oscillator that produces a highly temperature-dependent oscillation frequency acting as a temperature-to-frequency converter, and finally transfers the frequency-modulated signal to an external reader using back scattering. The power management circuits are eliminated in the designed sensor to arrive at a minimalistic design. For proper operation, a novel voltage regulator is developed that produces a relatively constant output voltage as the supply voltage of the ring oscillator for a large range of harvested input energy but allows the output voltage to change as a function of the temperature for added temperature sensitivity of the overall sensor. Power consumption of the proposed sensor is only $1.05~\mu\text{W}$ at room temperature, which enables continuous operation of the sensor from an incident energy of −16 dBm. The sensor is tested between −10 °C to 100 °C exhibiting a minimum sensitivity of 238 Hz/°C at −10 °C and a maximum sensitivity of 31.648 kHz/°C at 100 °C. The predicted temperature error is −2.6 °C to 1.3 °C using a two-point calibration within the range of 10 °C to 100 °C. With a conversion time of 25 ms, 0.046 °C (rms) resolution is achieved. Fabricated in IBM’s 130-nm CMOS technology, the proposed sensor occupies a die area of 0.23 mm2.