An Impedance Matching Method for LC Passive Wireless Sensors

This paper presents an impedance matching method for increasing the readout distance of an inductor-capacitor (LC) passive wireless sensor. When the impedance of the LC sensor system is equal to that of the measurement system, there is no reflection between the LC sensor system and the measurement system, which maximizes signal-to-noise ratio and readout distance. Complex impedance matching networks at the readout port of the LC sensor system have been analyzed and simulated. The matching capacitor at the readout port is required to change with the sensing capacitor in the LC sensor in order to maintain the same resonant frequency in both readout port and LC sensor. The matching resistor at the readout port has been analyzed and optimized. A varactor-based capacitor was inserted as the matching capacitor, which can be controlled by a DC voltage so that the resonant frequency of the readout port can be scanned within the full range of resonant frequency in the LC sensor. A capacitive humidity sensor constructed from a planar spiral inductor and an interdigital capacitor was used here to demonstrate the proposed impedance matching method. It shows that the readout distance under the impedance matching is 1.5 times as long as that without the impedance matching. The impedance matching analysis gives in-depth insight into the LC sensor system behavior, and the results presented here suggest that the impedance matching method may be utilized the long distance readout of the scaled LC sensor application based on the S ₁₁ measurement.