Wireless sensor based on modulated backscattering principle and LC oscillator

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Sähkötekniikan korkeakoulu | Master's thesis
Degree programme
EST - Master’s Programme in Radio Science and Engineering
This Thesis presents a passive wireless sensor utilizing the modulated backscattering principle. The sensor consists of an antenna, a rectifier, an oscillator, and a modulator. When the sensor is illuminated by a continuous wave (CW), the rectifier generates a DC supply voltage for the oscillator, which drives the modulator. As a consequence, the sensor produces a modulation to the reflected (CW). Sensing can be realized by designing the oscillator such that its frequency depends on the sensed variable. In this thesis, rectifier, oscillator, and modulator for the transponder are designed and implemented in practice. The operation of each part is analytically predicted using simplified models to anticipate the operation principle of the sensor. Through the analysis, sensor parameters such as rectified DC voltage, oscillator output voltage, power consumption by the oscillator and modulated reflected power of the sensor are predicted. As analytical equations indicate, the modulated reflected power of the sensor can be predicted as a function of input power of the sensor. It is found out that the rectified DC voltage depends on the diode parameters, input power and load impedance. Oscillator output voltage depends on the rectified DC voltage, oscillator circuit topology, and quality factor and characteristic impedance of the resonator. Moreover, power consumption by an oscillator can be made arbitrarily small by decreasing the capacitive divider ratio. Furthermore, modulated reflected power of the sensor depends on mixer diode parameters, input power of the sensor and output voltage of the oscillator. The operations of all parts are characterized by using simulation and measurement. Simulated and measured results show good agreement, which justifies the accuracy of the results. The developed characteristics are useful for predicting the sensor performance. The results show that the concept is feasible, but further development is needed to obtain a passive sensor with a large read-out distance and smaller power consumption.
Viikari, Ville
Thesis advisor
Rasilainen, Kimmo
backscattering, continuous wave (CW), modulator, oscillator, passive sensor, power consumption, read-out distance, rectifier, transponder
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