Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Piezoelectric crystal testing
Reexamination Certificate
1999-08-16
2001-09-18
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Piezoelectric crystal testing
C324S076310, C324S633000, C324S076290, C324S076390, C324S076410, C324S076440, C324S076460, C324S076490, C324S076510, C073S579000, C331S158000
Reexamination Certificate
active
06292002
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to frequency measurement and, more particularly, to frequency measurements of various quartz crystal resonant modes.
2. Description of the Prior Art
It is often desirable to determine the resonant modes of a quartz crystal arrangement where the frequency information may be used in scientific study, for example. Frequency characteristics of interest may include resonant frequencies, inharmonic overtones, and harmonic modes of the crystal arrangement.
Historically, methods of measuring resonant modes of a crystal include swept frequency sinusoidal analysis and crystal oscillator testing. The swept frequency sinusoidal analysis involves applying a sinusoidal signal to the crystal arrangement and sweeping the frequency of the sinusoidal input signal through a frequency range of interest. Based on the response of the crystal arrangement to the sinusoidal input, certain frequency characteristics can be obtained. The swept frequency sinusoidal analysis, however, is often tedious and overly time consuming, and often requires repetitive cycles of: providing an input signal to the crystal arrangement, changing the frequency of the input signal, measurement of the response of the crystal arrangement, comparison of the gathered information, and determining the input signal to be applied to the crystal arrangement for a subsequent measurement based on the current cycle measurement. Typically, for a high accuracy measurement very precise and expensive measurement equipment must be used. However, even when measurements are taken with the very precise and expensive equipment, errors may still arise due to human intervention during the analysis.
The crystal oscillator testing method for measuring a resonant mode of a crystal involves placing the crystal in an electronic oscillator circuit and measuring the oscillator signal frequency. Due to the resonant properties of a crystal, the electronic oscillator circuit will converge to the crystal's resonant frequency. While crystal oscillator testing is a reasonable method to determine the principal resonant frequency of a crystal, it is an undesirable method where additional resonant modes are to be measured.
OBJECTS OF THE INVENTION
It is one object of this invention to provide a method and apparatus to rapidly and accurately determine the frequency of a desired resonant mode characteristic of a crystal arrangement, or other two port device, under test without the use of complicated and expensive test equipment.
It is another object of this invention to provide a method and apparatus to rapidly determine the frequency of a desired resonant mode of a crystal arrangement, or other two port device, under test as part of a fully automated operation.
It is another object of this invention to provide a method and apparatus to rapidly determine the frequency of a desired resonant mode of a crystal arrangement under test while discriminating between several different modes of oscillation of the crystal.
It is another object of this invention to provide a method and apparatus for measuring multiple, closely spaced resonant frequencies of a crystal arrangement, or other two port device, under test.
It is another object of this invention to provide a precision, linear crystal-controlled oscillator utilizing an inexpensive crystal and a voltage controlled oscillator.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention. In accordance with one embodiment of the invention an apparatus and method are provided for measuring desired resonant modes of a crystal arrangement, or other two-port device. First, based upon calculated operating parameters related to a desired resonant mode of the crystal arrangement, the apparatus and method measure the passive response of the crystal arrangement to a sinusoidal input of a known frequency. The passive response of the crystal arrangement is measured by sweeping the frequency until the amplitude of the response is above a calculated threshold value. Second, the apparatus and method measure an error signal. The error signal represents the difference in frequency between the sinusoidal input and the desired resonant mode, generated, in part, as a result of the desired resonant mode of the crystal arrangement. Additionally, the apparatus and method fine tune, or closed-loop feedback adjust the frequency of the sinusoidal input such that the error signal rapidly converges to a predetermined value. When the error signal reaches the predetermined value, the frequency of the sinusoidal input is the frequency of the desired resonant mode of the crystal arrangement.
In accordance with the present invention, a crystal resonant frequency sensor includes a controller whose output is responsive to values measured at a minimum of two inputs, a voltage-controlled oscillator adapted to provide a signal whose frequency is responsive to a command signal, a phase shifter, a first and second multiplier and a first and second filter.
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Morgan Felix E.
Pringle, Jr. Ralph
Brown Glenn W.
Hamdan Wasseem H.
The B. F. Goodrich Company
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