The use of quartz-crystal oscillators as high-sensitivity microbalance sensors is limited by the frequency noise present in the circuit. To characterize the behavior of the sensors, it is not enough to determine their experimental sensitivity, but, rather, it is essential to study the frequency fluctuations in order to establish the sensor resolution. This is fundamental in the case of oscillators for damping media, because the level of noise rises due to the strong decline of the quality factor of the resonator. In this paper, a comparative study of noise and resolution is presented with respect to the frequency and the quality factor. The study has been made using four oscillators designed to be used in quartz-crystal microbalance sensors in damping media. The four circuits have been designed at increasing frequencies in order to improve the sensitivity or frequency change per unit of measurand. Also, the present theoretical resolution limit or best resolution achievable with a microbalance oscillator using an AT resonator is determined, since this does not depend on frequency. However, when operating in liquid, the damping of the resonator makes the resolution diminish due to a worsening of the quality factor. The relationship between the resolution limit and the frequency and characteristics of the liquid medium is determined. The resolution worsens when the density and viscosity of the liquid is increased. However, in this case, an increase in frequency implies a small increase in resolution. Therefore, we find that when working below the maximum quality factor, for similar values, the resolution can be improved by elevating the work frequency.

This work deals with the design of a high sensitivity DNA sequence detector using a 50 MHz quartz crystal microbalance (QCM) electronic oscillator circuit. The oscillator circuitry is based on Miller topology, which is able to work in damping media. Calibration and experimental study of frequency noise are carried out, finding that the designed sensor has a resolution of 7.1 ng/cm² in dynamic conditions (with circulation of liquid). Then the oscillator is proved as DNA biosensor. Results show that the system is able to detect the presence of complementary target DNAs in a solution with high selectivity and sensitivity. DNA target concentrations higher of 50 ng/mL can be detected.

In battery applications, particularly in automobiles, submarines and remote communications, the state of charge (SoC) is needed in order to manage batteries efficiently. The most widely used physical parameter for this is electrolyte density. However, there is greater dependency between electrolyte viscosity and SoC than that seen for density and SoC. This paper presents a Quartz Crystal Microbalance (QCM) sensor for electrolyte density-viscosity product measurements in lead acid batteries. The sensor is calibrated in H₂SO₄ solutions in the battery electrolyte range to obtain sensitivity, noise and resolution. Also, real-time tests of charge and discharge are conducted placing the quartz crystal inside the battery. At the same time, the present theoretical “resolution limit” to measure the square root of the density-viscosity product ( ) of a liquid medium or best resolution achievable with a QCM oscillator is determined. Findings show that the resolution limit only depends on the characteristics of the liquid to be studied and not on frequency. The QCM resolution limit for measurements worsens when the density-viscosity product of the liquid is increased, but it cannot be improved by elevating the work frequency.

A simple oscillator circuit for high resolution quartz crystal microbalance sensors is proposed for fluid media. It can solve the working problems of standard crystal controlled oscillators in damping media. The design is adapted to the conditions of strong damping that the quartz experiences, so that the oscillation stays in spite of the reduction of the resonator quality factor. The conditions for obtaining a stable frequency according to the resonator damping and the static capacity are studied. Experimental results confirming the success ot the circuit are reported.