Sensor measuring complex impedances

Sensorelement und Vorrichtung zur Messung komplexer Impedanzen sowie Verwendung der Vorrichtung

Abstract

Electrodes (29) are coated with an insulating layer (27). In comparison with their spacing (S), this layer is thin (d). Preferred Features: The insulating layer has high chemical stability. It is silicon carbide; its thickness, 0.2 - 2 mu m, is less than a quarter of the electrode spacing. The sensor is a surface acoustic wave (SAW) device. A capacitive electrode is applied to it. The sensor element is a surface acoustic wave sensor with interdigital electrodes on the substrate. One of the interdigital electrodes forms a measurement electrode pair. The measurement equipment includes analysis circuitry. Further variables are measured. The analysis unit is an oscillator circuit. The analysis unit voltage source produces two oscillatory signals of the same frequency. A current source controlled by one of them, produces an oscillatory current signal. An amplifier measures the signal dropped across the impedance to be measured, and is suitable to produce a corresponding oscillating voltage signal. A mixer takes this signal, combining it with the second of the two source signals cited with 0 deg or 90 deg phase shift, so that real and imaginary parts of the impedance can be determined. The amplifier has connections for positive and negative supply voltages supplied via a series resistance with connections to a voltage stabilizer and the amplifier output. Sensor line screening and buffer amplification are described. A signal switch changes over between individual sensors.

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