Readout circuit with improved sensitivity for contactless LC sensing tags
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AuthorCarvajal Rodríguez, Miguel Ángel; Escobedo, Pablo; Martínez Olmos, Antonio; Palma López, Alberto José
Contactless measurementReadout systemColpitts oscillatorPower measurementLC tag
Carvajal, M. A., Escobedo, P., Martínez-Olmos, A., & Palma, A. J. (2019). Readout circuit with improved sensitivity for contactless LC sensing tags. IEEE Sensors Journal, 20(2), 885-891.
SponsorshipThis work was supported by projects CTQ2016-78754-C2-1-R and FPA2015-67694-P from the Spanish Ministry of Economics and Competitivity and the European Regional Development Fund (ERDF).; and PI-0505-2017 within the biomedical and health sciences research, development and innovation projects in Andalucía.
In this work we present a novel technique to estimate the resonance frequency of LC chipless tags (inductor-capacitor parallel circuit) with improved sensitivity and linearity. The developed reader measures the power consumption of a Colpitts oscillator during a frequency sweep. The readout circuit consists of a Colpitts oscillator with a coil antenna, varactor diodes to change the oscillator frequency, analog circuitry to measure the power consumption and a microcontroller to control the whole system and send the data to a PC via USB. When an LC tag is inductively coupled to the oscillator, without contact, a maximum power peak is found. As shown by an experimental calibration using an LC tag made on FR4 substrate, the frequency of this maximum is related to the resonance frequency. Both parameters, power consumption and resonance frequency, present an excellent linear dependence with a high correlation factor (R2 = 0.995). Finally, a screen-printed LC tag has been fabricated and used as relative humidity sensor achieving a sensitivity of (-2.41 ± 0.21) kHz/% with an R2 of 0.946.