Abstract
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.
Highlights
INDUCTOR-capacitor (LC) resonant circuits can be used for low-cost, simple, passive and wireless sensing purposes [1]
This technique has been validated by comparison with impedance analyser measurements, providing improved sensitivity than our previous design and linear response in the whole measurand range when applied to relative humidity determination
The design of the reader coil and the printed LC tag for humidity measurements was the same used in our previous work [20], which was optimized via numerical simulation using Advanced Design System (ADS) simulator (Keysight Technologies, Santa Clara, CA, USA) and COMSOL Multiphysics® (Comsol Inc., Burlington, MA, USA)
Summary
INDUCTOR-capacitor (LC) resonant circuits can be used for low-cost, simple, passive and wireless sensing purposes [1]. Jacquemod et al proposed a reader architecture with low kcoupling factor compensation to improve the measurement distance, showing promising results up to 5 cm [29] Their design was based on a self-oscillating loop improved by an anti-resonance cancellation network for low coupling factors (long distances between the LC sensor and the readout coil). Changes in the maximum power consumption of the Colpitts oscillator due to changes in a coupled LC sensing tag have been accurately monitored by means of a frequency sweep This technique has been validated by comparison with impedance analyser measurements, providing improved sensitivity than our previous design and linear response in the whole measurand range when applied to relative humidity determination.
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