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Scheme for Maintaining Oscillations of at Cut Quartz Crystals in Liquid Media

IP.com Disclosure Number: IPCOM000038928D
Original Publication Date: 1987-Mar-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 51K

Publishing Venue

IBM

Related People

Gordon, JG: AUTHOR [+3]

Abstract

Quartz resonator apparatus is described which is capable of maintaining reliable and stable oscillations even when the quartz crystal is immersed in a liquid. The apparatus maintains oscillations, even when the resonator is damped by a liquid interface, by using increased feedback sufficient to compensate for these additional losses. The losses are represented as an increased effective series resistance in the resonator equivalent circuit, and these losses have been determined to be much greater, by a factor of one hundred, or more, in a liquid as compared to the series resistance in air. The design criteria are drastically altered by this damping, and the larger resistance makes the circuit more sensitive to shunting impedances, such as capacitive shunts and transistor loading.

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Scheme for Maintaining Oscillations of at Cut Quartz Crystals in Liquid Media

Quartz resonator apparatus is described which is capable of maintaining reliable and stable oscillations even when the quartz crystal is immersed in a liquid. The apparatus maintains oscillations, even when the resonator is damped by a liquid interface, by using increased feedback sufficient to compensate for these additional losses. The losses are represented as an increased effective series resistance in the resonator equivalent circuit, and these losses have been determined to be much greater, by a factor of one hundred, or more, in a liquid as compared to the series resistance in air. The design criteria are drastically altered by this damping, and the larger resistance makes the circuit more sensitive to shunting impedances, such as capacitive shunts and transistor loading. The feedback fraction is also altered by this increased resistance. Since the damping resistance can be larger than a hundred-fold, depending on the fluid properties, on the crystal mounting scheme and on the stresses due to deposits, the feedback fraction must be appropriately increased without excessive crystal loading. A specific circuit is shown in Fig. 1, in which a differential video amplifier 1 having a small signal gain of 400 is used. The two Schottky diodes 2 limit the signal amplitude of the feedback to less than 0.4 peak volt. The crystal 3 is an AT cut quartz crystal, and the feedback voltage is derived from the crystal current passing through the resistance R. The value of resistance R was chosen as the geometric mean between the series resistance of the crystal 3 in air and

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in the liquid. In one case the value was 220 ohms, but this value is notcritical, but serves to maintain reasonable feedback both when the crystal is in air and in liquid. The input impedance of the amplifier 1 is much larger than R, and the input capacitances are small enough so as to minimize their shunting effect on the crystal. One application for this quartz resonator apparatus is in a microbalance for measuring the rate of metal deposition in electroless plating baths. It can be demonstrated that a quantitative relationship exists between the mass of a metal deposited on the quartz oscillator and the change in frequency of oscillation in an aqueous solution, as shown in Fig. 2. The solid curve represents the results expected from theory, and the points represent the experimentally observed data. By the use of this relationship and apparatus, plating rates can be measured with improved accuracy, sensitivity and speed. Plating...