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

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 uW 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 mm
²
.