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 mu W at room temperature, which enables continuous operation of the sensor from an incident energy of -16 dBm. The sensor is tested between -10 degrees C to 100 degrees C exhibiting a minimum sensitivity of 238 Hz/degrees C at -10 degrees C and a maximum sensitivity of 31.648 kHz/degrees C at 100 degrees C. The predicted temperature error is -2.6 degrees C to 1.3 degrees C using a two-point calibration within the range of 10 degrees C to 100 degrees C. With a conversion time of 25 ms, 0.046 degrees 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).