Ultra low power CMOS medium frequency power collection and communication circuits for remote sensor nodes

Abstract

This thesis presents investigations into a system for the delivery of power and data to low power remote sensor nodes by means of a wide area magnetic inductive coupling, and describes the design and measurement of the essential receiver circuits fabricated in CMOS IC technology. Remote sensors applications require a long-life power supply and data information from the source in order to properly function. An inductive loosely coupled system, in which electromagnetic energy transmitted from the source is collected by many receivers, is a potential candidate for providing a power source and a channel for essential node control data. Moreover, the sensor network can be accurately synchronised by transmitting the modulated timing information carrier to the receivers which removes the risk of data collision, omitting the need for a high accuracy real-time clock in the remote node and its associated power drain. However, the received voltage at the secondary coil may not be sufficient to overcome transistor thresholds and activate the system and begin power transfer. Thus, a rectifier that can operate under the very low input voltage and a high quality factor inductor-capacitor receiver coil are required to maximise the operating range with a reliable start-up voltage. Since the bandwidth of the receiver coil is narrow due to its high quality factor characteristic, an accurate resonant frequency tuning system is needed to optimise the received voltage. Moreover, a phase-shift-keying modulation scheme is chosen for the data transmission since the carrier amplitude and frequency are ideally constant. However, the high quality factor behaviour of the receiver coil distorts the phase-shift-modulated carrier which leads to the failure of the data extraction if the receiver employs a conventional demodulator. A slow phase-shift-keying demodulator is needed to ensure that the data is extracted without an error when a high quality factor characteristic issued in the signal path.

The design presented in the thesis is an inductive loosely coupled energy harvesting and data demodulation receiver used for wide area wireless sensor network applications. A CMOS rectifier/receiver that can operate with the input voltage below the MOS threshold value with a medium frequency range and provide a DC output voltage delivered to the sensor node load for local storage in a battery or capacitor. In addition, the proposed receiver can extract data from slow phase-shift-keying modulation present on the powering signal. Furthermore, the resonant frequency tuning circuit is implemented to ensure that the receiver operating frequency is matched to the incoming frequency to maximise the received voltage.

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