No abstract

The resonant frequency of a thickness-shear mode resonator operated in contact with a fluid was measured with a network analyzer and with an oscillator circuit. The network analyzer measures changes in the device's intrinsic resonant frequency, which varies linearly with (ρη)(1/2), where ρ and η are liquid density and viscosity, respectively. The resonator/oscillator combination, however, responds differently to liquid loading than the resonator alone. By applying the operating constraints of the oscillator to an equivalent-circuit model for the liquid-loaded resonator, the response of the resonator/oscillator pair can be determined. By properly tuning the resonator/oscillator pair, the dynamic range of the response can be extended and made more linear, closely tracking the response of the resonator alone. This allows the system to measure higher viscosity and higher density liquids with greater accuracy.

dULl 7 1993 Albuquerque New Mexico 87185-5800 O _ T" IThe Lever o_illator has been specifically designed for design this type of o_illator with one side of the resonator at use with quartz resonator _nsors. The u_ of quartz resona-a.c. ground. With one side of the resonator grounded the tors as sensors is of particular interest and depending on the p,'u'asiticcapaciumces due to the fluid can be more easily _nsing environment, e.g., liquid, theoscillator design is both controlled; this is especi,'dly true if oxily one side of the critical and difficult due to the wide dyn,'unic nmge of resonator is used for liquid exl_sure.resonatorresistance lX_ssibledue to damping of theresonator. IStandard oscillator designs do not work well as sensor oscil-/ lators. An o_illator design will be pre_nted thai allowsboth ('" 1 frequency and loss (Rm) of the resonator to be determined < Lm i over a wide dynamic range of resonator loss. The Lever llm oscillator uses negative feedback in a differentialamplifier __._ Co configuration to actively and variably divide (or leverage) the Cm resonator impedance such that the oscillator can maintain the phase ,and gain of the lc_p over a wide range of resonator resistance._ Rm Intr(_uction I Figure 1. The quartz resonator mt_lel. Standard oscillator designs, like the Pierce or Colpitts type, are g(x_dfilr frequency source oscillator designs which provide low pha_ noi_ and high over-ali frequency stability. These designs do not necessarily work well for _nsor There are a few o,_illator configurations which operate applicatitms because of theirinherent pha._ and gain sensitiv-the resonator near I_,_ and have one side of the resonator ity to re.,amatorloss, Rm. Operating the resonator at f,_is a grounded. None of these configurations meet the design gotxl initial approach if one is to design mioscillator fi)r u._ goals of operating at 1_,_ of the resonator and outputting a with a high loss resonator. At fs the resonator is approxi-voltage proporticmalto the rescmator loss Rm, by way of an mately a real impedance and is at or near the minimum automatic level control (ALC)circuit. For in situ resonator impedance of the resonator. Figur,; 1 is the m(xlel for a bulk _nsors the Rm of the res(mator will have a wide dynamic wave resonator, with fs defined as the resonance of Lm and range. This dynamic range depends upon the resonator and Cm. Since a high loss resonator is not likely to be highly liquid properties but could be tens of ohms in air to _veral inductive at any frequency, it would be 'andesirableto try to thou_andohms in a viscous liquid. The resonator loss is a make the sensor operate atan inductive impedance rt,_ this will function of the viscosity-density product (1). Since quartz limit the dynamic range of the oscillator, lt is also desirable to resonator frequency shifts due to a given environment are

No abstract

The response of thickness shear mode (TSM) resonators in liquids is examined. Smoothsurface devices, which viscously entrain a layer of contacting liquid, respond to the product of liquid density and viscosity. Textured-surface devices, which also trap liquid in surface features, exhibit an additional response that depends on liquid de_isity alone. Combining smooth and textured resonators in a monolithic sensor allows simultaneous measurement of liquid density and viscosity.