Frequency characterization of quartz crystals

Techniques for determining a frequency profile of a quartz crystal in real time. Quartz crystals are subjected to a series of temperature cycles at various temperature rates and the crystal frequencies, crystal temperature parameters, and the temperature rates are monitored as the crystal is subjected to the temperature cycles. The monitored frequencies are grouped correlated with the monitored temperature parameters and temperature rates. A system for determining the frequency of a quartz crystal includes a processor adapted to perform the frequency profiling techniques.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to the field of quartz crystals used as highly stable frequency standards (such as clocks). More particularly, the invention relates to techniques for profiling or characterizing the frequency output of crystal-based oscillators with reduced deviations in frequency due to environmental effects.

2. Background Art

Timing of operation of electronic devices, particularly digital devices, requires an accurate, frequency stable clock signal. Many such electronic devices are subjected to variations in ambient temperature during their operation. As is well known in the art, changes in ambient temperature affect the frequency of a typical crystal.

While quartz oscillators are considerably more stable when compared to other types of oscillators, their frequency output is known to exhibit some drift under rapid temperature variations. The effect of stresses on quartz crystals is well known and exploited in the design of quartz based stress and pressure sensors. Due to the low thermal conductivity and the anisotropic properties of quartz, heating and cooling crystals is known to cause stresses in the crystal, which affects the frequency. See, Bottom, Virgile E.Introduction to Quartz Crystal Unit Design,New York: D. Van Nostrand, 1982. For this reason, it is generally not recommended to subject quartz crystals to rapid temperature gradients.

In conventional applications the frequency deviations of quartz crystals due to temperature are profiled or characterized during manufacture and compensated for in real time. Rapid temperature fluctuations in the crystal's environment and changing temperature rates, also referred to as temperature gradients, cause the crystal frequency to deviate from the characterization. Though the varying temperature rates may last for a brief period, their effects can last for long periods of time, causing measurement errors.

In order to achieve greater frequency stability, several methods have been proposed to account for these deviations. One approach is to place the crystal in a temperature controlled chamber, which will keep the crystal at a constant temperature and prevent any deviation in frequency. See, for example, U.S. Pat. Nos. 5,917,272, 5,729,181, 5,180,942, 4,586,006 and 3,619,806. Another approach taken to compensate for the deviations in frequency due to temperature is to use a voltage-controlled oscillator of which the frequency can be adjusted by changing the voltage at the control input. In these designs, the temperature at the crystal is measured and used to digitally compute a correction voltage to be applied to the voltage-controlled oscillator. See, for example, U.S. Pat. Nos. 5,668,506, 5,473,289, 5,214,668, 5,170,136, 5,081,431, 4,922,212, 4,746,879, 4,427,952 and 4,380,745.

One problem with using a temperature sensor and measuring the temperature outside the crystal is that there is a time lag between the actual crystal temperature (at the quartz plate) and the outside where the temperature is measured. This causes the oscillators to be slow in responding to a change in temperature, introducing errors. A proposed solution to this problem is to have the crystal oscillate in two modes simultaneously, where one of the two modes is temperature sensitive while the second mode is relatively stable with temperature. The temperature sensitive mode is used to obtain the temperature at the crystal itself and then used to compensate for minor deviations with temperature in the stable mode. See, for example, U.S. Pat. Nos. 5,525,936 and 4,079,280. In spite of the very accurate measurement of temperature in these designs, high temperature gradients in the environment still introduce errors. Modern crystals are also cut at special angles, such as the SC cut, in an attempt to minimize frequency deviation due to temperature.

Another method that can be used to minimize problems due to fluctuating temperature rates is to place the crystal in a temperature controlled chamber. See, for example, U.S. Pat. No. 6,606,009 (assigned to the present assignee). However, this option entails higher power consumption, which can be a disadvantage in certain applications. Thus a need remains for improved techniques to account for and minimize frequency deviation in crystal-based oscillators due to environmental variations.

SUMMARY OF INVENTION

An aspect of the invention provides a method for determining a frequency profile of a quartz crystal. The method comprises subjecting the quartz crystal to temperature cycles at various temperature rates; monitoring the crystal frequencies, a crystal temperature parameter, and the temperature rates as the crystal is subjected to the temperature cycles; and grouping the monitored frequencies correlated with the monitored temperature parameters and temperature rates.

An aspect of the invention provides a method for determining a frequency of a quartz crystal. The method comprises subjecting the quartz crystal to temperature cycles at various temperature rates; monitoring the crystal frequencies, a crystal temperature parameter, and the temperature rates as the crystal is subjected to the temperature cycles; grouping the monitored frequencies correlated with the temperature parameters and temperature rates; determining the temperature and a temperature rate of the crystal; and relating the determined crystal temperature and temperature rate to the grouped frequencies to determine the crystal frequency.

An aspect of the invention provides a method for determining a frequency of a quartz crystal. The method comprises determining a temperature of the quartz crystal; deriving a temperature rate from the determined crystal temperature; and relating the crystal temperature and temperature rate to a data set characterizing a correlation between the crystal frequency, temperature, and temperature rates to determine the crystal frequency.

An aspect of the invention provides a system for determining a frequency of a quartz crystal. The system comprises a crystal having a frequency output related to a temperature of the crystal; and a processor adapted to calculate a crystal frequency from a measured temperature parameter of the crystal, a temperature rate of the crystal, and observed frequencies of the crystal correlated with observed temperature parameters and temperature rates of the crystal.

DETAILED DESCRIPTION

In quartz crystal oscillator applications that require significant frequency stability, the temperature dependency of the frequency is typically profiled or characterized during manufacturing and captured in the form of a polynomial or look up table. During characterization, a crystal is subjected to a temperature cycle while the frequency and temperature are monitored. A temperature cycle involves heating from the lowest to highest operating temperature and then cooling down from highest to lowest temperature. The repeatability of the crystal frequency response is crucial to the success of high stability applications. If the crystal were perfect, the frequency response during heating would theoretically match perfectly with the frequency response during cooling. In reality, however, a crystal's responses do not match perfectly. This effect, sometimes referred to as hysteresis, causes the response during heating to be slightly different from that during cooling. This effect is referred to as temperature rate/gradient effects in this disclosure as a strong dependency of this effect on the rate of heating or the temperature rate has been observed.

An example of a basic quartz crystal device that may be used to implement various aspects of the invention is shown generally inFIG. 1. A quartz plate or disc14is attached to mounting clips/electrical leads16. The disc14is disposed within a housing10and sealed therein by an insulating layer18(e.g. glass layer). The housing10is preferably evacuated to form a vacuum area12for the quartz disc14and surroundings. Electrical connections to electrodes on the disc14are made via the leads16passing through the insulating layer18. AlthoughFIG. 1shows one sample quartz crystal device, it will be appreciated by those skilled in the art that there are many standard package styles/configurations used in mounting quartz crystals. Further description of quartz crystal packages is found in Griffith, James E., “Development And Advancements in SC-Cut Crystals”, RF Expo EAST, 1994, (http://www.corningfrequency.com).

FIG. 2shows a more detailed view of the quartz disc14as viewed from above. The disc14has two metal electrodes, one electrode24on the top surface and the other22on the bottom surface, to provide the electrical stimulus to make the disc vibrate. The electrodes22,24are disposed on the disc14by means well known in the art. The disc14housing10is metallic, which is typical for conventional crystal packages.

When one considers the characterization cycle where the crystal is heated, one can expect the metal housing10to heat first and then the disc14. In this situation, since the area inside the housing10is a vacuum, the strongest heat flow is expected through the mounting clips/leads16connected to the electrodes22,24as they are made of metal, which conducts heat well. Therefore, when one considers the temperature distribution of the quartz disc14, one can expect the immediate areas close to the connected leads16to get hotter while areas further away from the leads remain relatively cooler since quartz is generally a poor heat conductor. Note that the mounting clips16not used as leads for the electrodes22,24may be non-metallic in some designs. One can expect the hottest disc14areas to expand most due to thermal expansion and the colder areas to expand less. This type of mismatch in expansion is likely to induce mechanical stresses, causing changes in the vibrating frequency.

During the cooling part of the cycle, the housing10exterior is colder relative to the disc14and heat flows in the opposite direction through the leads16. Thus in this situation, the area further away from the leads16will be hot and expanded while the area closer to the leads will be cooler and contracting. This reversal in stress states affects the crystal, causing frequency shifting in one direction during heating and in the opposite direction during cooling. These stresses induced by non-uniform temperature distributions are key factors in quartz crystal frequency shifts, producing frequency gradient effects.

As previously discussed, conventional methods of compensating for deviations in quartz oscillator frequency output due to temperature are characterized as
f=f(T),  (1)
where f represents frequency and T the temperature. Manufactured crystals are subjected to a temperature cycle while the frequency and temperature are measured. This data is used to compute Equation (1) by optimization. This function is typically represented as a polynomial:

f⁡(T)=∑l=0n⁢al⁢Tl,(2)
and the coefficients are computed by optimization (polynomial fit to the data) using the characterization data. These coefficients are typically stored and used to compute the actual frequency of the oscillator by measuring the temperature.

Techniques of the present invention account for temperature gradient effects on the crystal by performing a two-dimensional characterization where the two dimensions are a temperature parameter and temperature rate. The temperature parameter may be any parameter representative of temperature. In one embodiment the temperature parameter is the ratio of frequencies (Fb/Fc) as described in U.S. Pat. No. 6,606,009 (incorporated herein in its entirety by reference), with the temperature rate being captured by the time derivative of the parameter. In one process of the invention, the characterization involves subjecting the quartz crystal14to multiple temperature cycles with varying temperature rates. The crystal frequency is then characterized as a function of both the temperature parameter and temperature rate as follows:

f=f⁢(⁢T,T.⁢),T.=ⅆTⅆt,(3)
where f represents frequency, T represents the temperature or any parameter representing temperature,
{dot over (T)}
represents a time derivative of T, and t represents time. The characterization can be represented by a polynomial or look up table which may be used in real time to compute the crystal frequency.

In the two-dimensional approach of the invention, a crystal is subjected to a series of temperature T cycles1,2,3,4,5,6of varying temperature rates, as shown inFIG. 3. During these cycles the state of the crystal can be considered as going through the curve shown inFIG. 4on a plot of temperature T versus temperature rate
{dot over (T)}
for a simple case where the heating and cooling rates are the same. Looking at temperature cycle1inFIG. 3, the temperature is increasing at a constant rate (e.g. 20 degrees/hour), thus the corresponding curve1
(rate T′)
inFIG. 4is a positive constant as the temperature T increases. In the cooling cycle2, curve2(FIG. 4) remains constant as the temperature T decreases, but it is now a negative rate. For the next cycle3, the temperature rate is higher as represented by curve3inFIG. 4, and so forth.

As the crystal14is subjected to the temperature cycles, the frequency, temperature parameter, and temperature rate are monitored and recorded. This characterization data can be graphed to define the shape of a surface within Cartesian three-dimensional space using a standard mathematical function of two real variables which assigns a unique real number or point z=f(x, y) to each ordered pair (x, y) of real numbers in the recorded data set. In this case, the ordered pair consists of the monitored temperature parameters T and temperature rates
{dot over (T)}.

As shown inFIG. 5, the crystal frequencies can be pictured as a set of points
(T,{dot over (T)})
in the xy plane and the graph of the frequency function as the surface
f=f(T,{dot over (T)}).
Thus as the point
(T,{dot over (T)})
varies in the data set domain, the corresponding point (x, y, z)=
(T,{dot over (T)},f(T,{dot over (T)}))
varies over the surface. Any suitable software may be used to process the data set and plot the surface as known in the art. Interpolation or extrapolation techniques known in the art may be used to derive missing points in the surface
f=f(T,{dot over (T)}).
Once the surface is generated, it can be used in real time to compute the frequency more accurately by computing the temperature parameter T and temperature rate
{dot over (T)}

Undesired gradient effects can also be reduced or eliminated by making the crystal14temperature distribution more uniform.FIG. 6shows another embodiment of the invention. In this embodiment a dummy plating26is disposed on the quartz disc14surface to improve heat conduction across the disc. One or both sides of the disc14may be equipped with the plating26. Any suitable heat conductor may be used for the plating26material (e.g. metal, which is good heat conductor). The plating26may be disposed on the disc14via any suitable means known in the art (e.g. electroplating, vapor deposition, etching, adhesives, etc.). Sufficient clearance should be left between the dummy plating26, the mounting clips/leads16, and the electrodes22,24to prevent electrical shorts. Some embodiments may be implemented with bigger electrodes22,24to cover a larger portion of the quartz disc14surface (not shown).

It will be appreciated by those of ordinary skill in the art that the present invention is applicable to, and can be implemented in, any field where quartz crystal oscillators are used as frequency standards (e.g. in apparatus for use in outer space, automobiles, etc.). While not limited to any particular application, the present invention is suitable for subsurface applications, where rapid temperature variations are encountered.

FIG. 7shows another embodiment of the invention. A quartz crystal oscillator48is shown mounted in a downhole logging tool28disposed in a borehole30that penetrates an earth formation. The oscillator48is housed within a thermally insulated chamber50to reduce heat flow to the crystal during heating and cooling. The chamber50provides thermal insulation through the use of conventional insulating materials or by using a dewar flask as known in the art and described in U.S. Pat. No. 6,606,009. The tool28also includes a multi-axial electromagnetic antenna19, a conventional source/sensor44array for subsurface measurements (e.g., nuclear, acoustic, gravity), and electronics42with appropriate circuitry. The tool28is shown supported in the borehole30by a logging cable36in the case of a wireline system or a drill string36in the case of a while-drilling system. With a wireline tool, the tool28is raised and lowered in the borehole30by a winch38, which is controlled by the surface equipment32. Logging cable or drill string36includes conductors34that connect the downhole electronics42with the surface equipment32for signal and control communication. Alternatively, these signals may be processed or recorded in the tool28and the processed data transmitted to the surface equipment32.

It will also be apparent to those skilled in the art that this invention may be implemented by programming one or more suitable general-purpose microprocessors. The programming may be accomplished through the use of one or more program storage devices readable by the processor and encoding one or more programs of instructions executable by the processor for performing the operations described above. The program storage device may take the form of, e.g., one or more floppy disks; a CD ROM or other optical disk; a magnetic tape; a read-only memory chip (ROM); and other forms of the kind well-known in the art or subsequently developed. The program of instructions may be “object code,” i.e., in binary form that is executable more-or-less directly by the processor; in “source code” that requires compilation or interpretation before execution; or in some intermediate form such as partially compiled code. The precise forms of the program storage device and of the encoding of instructions are immaterial here. Thus these processing means may be implemented in the surface equipment32, in the tool28, or shared by the two as known in the art.

An embodiment of the invention relates to a process for determining a frequency profile of a quartz crystal.FIG. 8outlines the process. First the quartz crystal is subjected to temperature cycles at various temperature rates (step100). This step may be performed during manufacture of the crystal or at any suitable location (e.g., a laboratory, a field location, etc.). Next, the crystal frequencies, a crystal temperature parameter, and the temperature rates are monitored as the crystal is subjected to the temperature cycles (step105). Then a grouping is done of the monitored frequencies correlated with the monitored temperature parameters and temperature rates (step110). The data grouping may be performed using processor means or any other suitable means known in the art.

FIG. 9is a flow chart illustrating a process for determining a frequency of a quartz crystal according to the invention. The process begins by subjecting the quartz crystal to temperature cycles at various temperature rates (step200). At step205, the crystal frequencies, a crystal temperature parameter, and the temperature rates are monitored as the crystal is subjected to the temperature cycles. Then the monitored frequencies correlated with the temperature parameters and temperature rates are grouped (step210). At step215, the crystal temperature and a temperature rate of the crystal are determined. The temperature and rate determination is performed using any means known in the art and suitable for the particular environment. Finally, the determined crystal temperature and temperature rate are related to the grouped frequencies to determine the crystal frequency (step220). This association is performed as described herein using microprocessor means or any other suitable means known in the art.

FIG. 10is a flow chart illustrating a process for determining a frequency of a quartz crystal in real time according to the invention. The process begins by determining a temperature of the quartz crystal (step300). The crystal temperature may be determined using any suitable means known in the art and appropriate for the particular crystal environment. A temperature rate is then derived from the determined crystal temperature (step305). At step310, the crystal frequency is determined by relating the crystal temperature and temperature rate to a data set characterizing a correlation between the crystal frequency, temperature, and temperature rates. The data set is compiled as described herein. For the purposes of this specification it will be clearly understood that the word “comprising” means “including but not limited to”, and that the word “comprises” has a corresponding meaning.