Ultra-Low Power Wireless Sensor Circuits for IOT Applications

Wireless sensors, which are responsible for local data acquisition, processing and communication, play an important role in Internet of Things (IoT) applications. This research focuses on two basic components in wireless sensors, i.e., the low-power frequency tunable wireless receiver and the power management unit (PMU) for autonomous operation. In IoT applications, different sensors may need to operate in different frequency bands in order to meet environment constraints and industrial/medical standards. Thus, it is highly desirable to design a frequency configurable wireless receiver that provides flexibility in operation frequency. A 4-path filter based frequency shift keying (FSK) receiver is proposed to meet such a need, where the carrier frequency can be adjusted without changing the circuit. In addition, the proposed receiver requires no low-noise amplifier (LNA), which boosts the power efficiency. Frequency synthesizer is critical in FSK transceiver as it provides an accurate reference frequency. Based on 4-path mixer, a novel two-step calibration frequency synthesizer structure is proposed for low power consumption and wide locking range. Measurement results show that the proposed receiver achieves an energy per bit as 74pJ/bit with 2.5Mbps data rate and 184W power consumption. Post-layout simulation results show that the proposed frequency synthesizer has a figure of merit (FOM) value as 1.4W/MHz with 220MHz tuning bandwidth and 305W power consumption. Autonomous operation is another requirement for the sensors in many IoT applications, such as wearable sensors. Energy harvester is commonly used for autonomous sensors, where a PMU with low start-up voltage is necessary. To meet such a requirement, a novel controller for PMU is proposed to boost the power efficiency under very low load current. The proposed PMU can be started up with input voltage as low as several tens of millivolts. The novel controller costs little power and the overall efficiency is increased. Also, a hardware efficient maximum power point tracking (MPPT) algorithm which is suitable for energy source with fixed internal resistance is proposed. Measurement results show that the proposed system has a low controller power as 3.6W and the overall conversion efficiency is 83.9%.