Design of Self-Powered Wireless Sensors Using Efficient RF Energy Harvesters

• Author / Creator

  Saffari,Parvaneh

• The “Internet of Things” (IoT) promises to extend internet connectivity to billions of devices. A large number of these devices, such as wireless sensors, that will be wirelessly connected to the internet will not have a wired connection to the electricity grid relying on the energy stored in the batteries to power themselves. The battery power source is a bottleneck for the scaling of a wireless sensor network to thousands or millions of nodes due to its lifetime and capacity limitations. Energy harvesting presents a viable solution for powering at least some wireless sensors and devices. Ambient sources of energy such as solar, vibrational, thermal and radio frequency (RF) can be harvested and utilized. RF energy harvesting is one of the most popular energy scavenging methods because of its ever-expanding availability, integration capability, and compatibility with wireless networks. The major limitation of harvesting RF energy is the limited amount of the energy that can be scavenged due to the path loss, rapid attenuation of signals over distance, the low power efficiency of RF-DC converters, and the limited maximum allowed transmitted signal strength as restricted the regulations. To build a wireless sensor entirely powered by harvested RF energy, it is crucial to enhance the efficiency of RF rectifiers and to minimize the power consumption of the sensor circuitry and wireless transmitter that are required to transmit the sensed data to a reader.
  In this Ph.D. work, we addressed sensor challenges by designing a highly efficient RF energy harvester and an ultra-low-power wireless temperature solely powered by RF energy harvesting.
  In this dissertation, we first describe a wide input range, 4-stage threshold voltage compensated RF-to-DC power converter, designed to efficiently convert RF signals to dc voltages by applying an optimum compensation voltage produced by subthreshold auxiliary transistors. The proposed optimally compensated rectifiers achieve higher efficiency over a wider input power range compared to other threshold voltage compensation circuits, where the level of the compensation is limited by the circuit structure and varies with input power. This proposed compensation technique can be applied to a rectifier chain with a relatively small number of stages. Designed and implemented in 130-nm CMOS technology, the proposed rectifier exhibits a measured PCE of above 20% over the 8.5-dB input power range while driving a 1-MΩ load resistor at 896-MHz.
  To prove the feasibility that a wireless sensor can be entirely powered by RF energy harvesting, a fully integrated RF-powered temperature sensor with non-intermittent operation is presented. The proposed 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 the added temperature sensitivity of the overall sensor. The power consumption of the proposed sensor is only 1.05 µW at room temperature, which enables continuous operation of the sensor from an incident energy of −16 dBm. Fabricated in IBM’s 130-nm CMOS technology, the proposed sensor occupies a die area of 0.23 mm2.

• Subjects / Keywords

  • RF Energy Harvester
  • Integrated Sensor
  • Rectifier

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-x2xt-xg93

• License

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