Multiple Quartz Crystals Connected in Parallel for High-Resolution Sensing of Capacitance Changes

Abstract

We present a new highly sensitive, low-value capacitance sensor method that uses multiple quartz crystals connected in parallel inside the oscillator. In the experimental setup, the measured (sensible) reactance (capacitance) is connected in parallel to the total shunt capacitance of the quartz crystals, oscillating in the oscillator. Because AT-cut crystals have a certain nonlinear frequency–temperature dependence, we use the switching mode method, by which we achieve a temperature compensation of the AT-cut crystals’ frequency–temperature characteristics in the temperature range between 0–50 °C. The oscillator switching method also compensates for any other influences on the frequency of the oscillator, such as ageing of the crystals and oscillator elements, supply voltage fluctuations, and other parasitic impedances in the oscillating circuit. Subsequently using two 50-ms-delayed switches between the measuring and reference capacitors, the experimental error in measuring the capacitance is lowered for measurements under a dynamic temperature variation in the range of 0–50 °C. The experimental results show that the switching method, which includes a multiple quartz connection and high-temperature compensation improvement of the quartz crystals’ characteristics, enables a sub-aF resolution. It converts capacitance changes in the range 10 zF–200fF to frequencies in the range 4 kHz–100 kHz.

Assessing Impedance Analyzer Data Quality by Fractional Order Calculus: A QCM Sensor Case Study

“…The use of the quartz crystal microbalance (QCM) sensor in liquid media [29][30][31] has brought great benefits in expanding the range of applications [32][33][34], especially in biological media [35]. A significant experimental effort was required to monitor the QCM sensor's series resonance frequency and damping parameters using an active method (oscillator) [36][37][38] or passive interrogation, such as as ring-down methods [39][40][41] in high-viscosity liquid media.…”

Measurements of Small Frequency Differences by Dual Mode 4 MHz Quartz Sensors

“…However, when we measure the frequency difference (the crystals in the oscillator act as sensors), the measurement error depends on the chosen method [ 30 ]. If changes of the equivalent circuit of the quartz crystals are in the aF or zF region (in the case of the capacitive effect) or in the pH region (in the case of an inductive effect), changes in the resonant frequency, which are typical in measurements of a mechanical displacement, nanopositioning, eccentric motion, strain sensing, dielectric properties of liquids and density of liquids, low pressure, etc., are very low [ 4 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 ]. It is of crucial importance that these small changes of the resonant frequency are measured as accurately as possible, before they are transformed into the measured quantity.…”

Effect of Load on Quartz Crystal Microbalance Sensor Response Addressed Using Fractional Order Calculus

“…This scenario, described by Sauerbrey’s equation [ 25 , 26 ], is not affected by coating one or both electrodes of the QCM sensor with a thin sensing layer if it is firmly bound, and its mass does not change the linear dependence of the serial resonant frequency with the deposited mass. In this situation, an experimental setup that consists of an oscillator with a QCM sensor in the loop [ 29 ] and a frequency meter is commonly used. This experimental configuration might be more efficient if the parallel capacitance was compensated to eliminate the discrepancy between the serial resonant frequency of the QCM sensor and the frequency of the oscillator output.…”