Wireless Readout of Passive LC Sensors

This paper reports simple yet precise equations for automated wireless measurement of the resonance frequency,
$Q$
-factor, and coupling coefficient of inductively coupled passive resonant
LC
circuits. This allows remote sensing of all physical and chemical quantities that can be measured with capacitance transducers. Formerly reported front-end circuit concepts for wireless sensor readout, i.e., phase dip measurement and the dip meter, are subsequently discussed. It is shown that, due to fundamental system limitations, the formerly reported circuit concepts are not applicable if the distance between the sensor and the readout electronic circuit becomes too small, resulting in large coupling coefficients. Therefore, we present an improved concept for an analog front-end circuit of the readout system that overcomes these limitations and hence allows wireless sensor readout under a wider range of operating distances.