On Optimization and Read-out Resolution of the Zero Power Sensor

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Journal Title
Journal ISSN
Volume Title
Sähkötekniikan korkeakoulu | Master's thesis
Date
2012
Major/Subject
Radiotekniikka
Mcode
S-26
Degree programme
Language
en
Pages
[8] + 59 s.
Series
Abstract
Wireless sensors are emerging these days. In certain applications, they reduce installation complexity and maintenance costs. However, there are still limitations with the current passive wireless sensors such as a short detection distance and limited parameters to monitor. In this thesis work, we further develop a new concept called Zero Power to overcome challenges related to the state-of-art of passive wireless sensors. Zero Power sensor is claimed to exhibit a long read-out distance and being capable to monitor several quantities. The concept utilizes the intermodulation communication principle, in which the sensor rejects a signal which contains sensing information at the intermodulation frequency while illuminated by the reader device at two closely located frequencies. A wired sensor prototype is fabricated. The sensitivity of the sensor is experimentally characterized and the achievable read-out resolution, which defines the smallest detectable relative change in the sensor capacitance, is theoretically predicted for the first time. After the first prototype, the optimization theory of the sensor circuit is established. Based on the theory, the sensor circuit is optimized to achieve the best resolution. A conjugate matched antenna for the sensor is utilized to reduce the complexity of the sensor circuit and the energy consumption. An optimized sensor with an antenna is manufactured. The sensitivity of the optimized sensor is experimentally characterized and the achievable read-out resolution of the optimized sensor is estimated. Finally, a wireless ECG (Electrocardiography) measurement is demonstrated with the optimized sensor. According to the experimental results, the sensor can offer 1 % relative resolution (1 % change in the sensor capacitance can be detected) at 13 m distance.
Description
Supervisor
Räisänen, Antti
Thesis advisor
Viikari, Ville
Keywords
conjugate matching, intermodulation response, mixers, RFID, read-out resolution, sensitivity, wireless sensor
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