Patent Publication Number: US-2002005765-A1

Title: Digital indirectly compensated crystal oscillators

Description:
RELATED APPLICATION  
     [0001] This application claims priority to, and hereby incorporates by reference in its entirety, U.S. Provisional Patent Application No. 60/190,270 filed Mar. 17, 2000. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] This invention relates to crystal oscillators. More particularly, the invention relates to digital indirectly compensated crystal oscillators.  
       [0004] 2. Description of the Related Art  
       [0005] The individual components in certain airborne navigation and landing systems, such as global positioning systems, are usually equipped with clock generators that provide time control of the internal operations or procedures of system components. To achieve frequency synchronism between the individual system components, clock information generated by a central, high-precision clock device is communicated to the system components and is used for synchronization of the clock generators. Such frequency synchronization devices are known as phase frequency control circuits, or phase locked loop (PLL) devices. A PLL device typically includes a voltage-controlled oscillator (VCO) and a phase comparator with a following filter. The comparator compares phases of the output signal of the VCO to the clock information and controls the VCO depending on the result of the comparison. The VCO typically comprises a piezoelectric crystal that is used to provide a stable output frequency.  
       [0006] As is well known, piezoelectric crystals derive functionality from the fact that they deform slightly in the presence of an electric field. Generally, however, the operating frequency of a piezoelectric crystal varies as a function of temperature. Hence, changes in ambient temperature cause temperature fluctuations in the crystal, which in turn produce deviations in the output frequency of the crystal. The frequency deviations follow temperature response curves that can be determined for each unique crystal. This is commonly known as the frequency-temperature (FT) response of the crystal. FIG. 1 shows the Bechmann curve for a typical quartz crystal. The FT response of a crystal is partly determined by the type of crystal section, i.e., the angle relative to a crystal axis, at which the crystal is cut from a source quartz crystal. Many current radiotelephones, for example, use a so-called “AT-cut” crystal for the crystal oscillator. In general, the AT-cut crystal has the most stable frequency over temperature.  
       [0007] Because the FT response of a crystal is generally an undesirable effect, a temperature compensating apparatus is normally used to stabilize the crystal&#39;s frequency output. Two basic techniques are commonly used to accomplish this. One technique involves enclosing the crystal within an oven to maintain the crystal at a constant temperature and thereby keep its frequency output stable. A crystal oscillator compensated in this manner is known as an oven controlled crystal oscillator (OCXO). The second technique involves applying a temperature varying voltage across a voltage variable capacitor, which controls the resonant frequency of the crystal. When the temperature-varying voltage is properly derived, the voltage variable capacitor tunes the crystal in such a manner as to maintain its actual frequency output near the desired frequency output over the required temperature range. A crystal oscillator controlled in such a manner is called a temperature compensated crystal oscillator (TCXO).  
       [0008]FIG. 1B is a functional block diagram of an OCXO well known in the relevant technology. The crystal  112  in an OCXO is typically enclosed in an oven-like structure  114  that is heated by a heating device to maintain the temperature of the crystal  112  at a predetermined temperature, which is usually the temperature at which the crystal&#39;s  112  resonant frequency is most stable. A temperature sensing mechanism  118  is frequently used in a closed loop feedback system to control the amount of heat input to the oven in order to keep the temperature and, hence, the frequency of the crystal  112  stable over time. Typically, OCXOs are provided with separate fixed or variable components for both tuning the crystal oscillator to a desired frequency, and setting the oven at a desired temperature. Thus, the purpose of an OCXO is to minimize frequency change over temperature by maintaining the crystal within a stable thermal environment. An OCXO, however, requires a large volume of space and consumes substantial amounts of power.  
       [0009] The most popular crystals for OCXO applications are AT- and SC-cut quartz crystals, which are well known in the relevant technology. The SC-cut crystal is generally unsuitable for use in TCXO designs because it is difficult to tune; however, it works very well in OCXO applications. Because the SC-cut crystal has lower noise characteristics relative to a comparable AT-cut crystal, applications that require low noise levels typically use the SC-cut crystal; however, such designs incur the costs associated with the larger size and power requirements of the OCXO design required to stabilize the SC-cut crystal.  
       [0010] For many quartz crystals, the temperature at which the quartz must be kept to stabilize its frequency is relatively high. A SC-cut crystal, for example, is known to have a very stable output frequency near 80° C. Unfortunately, maintaining such a high temperature in an ovenized environment consumes large amounts of power; additionally, the relatively high temperature results in a long temperature settling time (i.e., the time it takes for the temperature inside the oven to stabilize). Moreover, other temperature-sensitive semiconductor components located close to the oven might require thermal isolation from the oven to protect them from the heat transferred from the OCXO that can adversely affect their performance. Another problem frequently encountered with OCXOs is that the persistent high temperature of the crystal causes premature performance degradation. The higher temperature decreases the mean time between failures (MTBF) of the device, and degrades aging performance. This premature performance degradation increases the maintenance costs of systems having OCXOs.  
       [0011]FIG. 1C is a functional block diagram of a TCXO  140  well known in the relevant technology. TCXOs are commonly found in electric communication devices, such as cellular phones and wireless radios, which require stable operating frequencies and low power consumption. Unlike OCXO designs, a TCXO  140  application does not attempt to maintain a stable thermal environment for the crystal  142 . In a TCXO  140 , compensation for frequency deviations of the crystal  142  is accomplished by varying the voltage  144  applied to the voltage variable capacitance  150  regulating the crystal  142 , rather than maintaining the crystal  142  at a nearly constant temperature. TCXOs are known to include analog and digital types, each utilizing several components. A typical analog TCXO includes a piezoelectric element, capacitors, inductors, resistors, etc. A typical digital temperature-compensated crystal oscillator (DTCXO) includes a piezoelectric element, an integrated circuit, and capacitors.  
       [0012] In analog TCXO applications, usually the crystal is thermally coupled to a temperature sensor  146  of a temperature measuring device which creates an analog voltage that is directly applied to a tuning device  148 , e.g., the voltage control input of the oscillator  142 . In DTCXO applications, the analog voltage output of the temperature sensor may be digitized and supplied to a memory. The memory stores in digital form the voltage values required for compensation at the measured temperature. Each digitized measured temperature value has a digital compensation voltage value assigned thereto. After a digital to analog conversion, the voltage compensation value is fed to the voltage control input of the crystal as an analog compensation voltage and effects a correction of the temperature-conditioned frequency deviations.  
       [0013] Generally, TCXOs are used to provide a frequency that is stable to within five parts per million (5 ppm) or less. Higher stability requires more complexity in a TCXO design. Analog circuits become ungainly in high stability oscillators because they require additional components. Consequently, digital temperature compensated crystal oscillators (DTCXOs), which incorporate complex integrated circuitry, are being used increasingly in applications requiring 2 ppm stability or better. An example of such a DTCXO is described in U.S. Pat. No. 5,691,671, issued to Bushman, entitled “Method and Apparatus for a Crystal Oscillator using Piecewise Linear Odd Symmetry Temperature Compensation,” and which is incorporated by reference herein in its entirety. Typically, the DTCXO includes a memory containing predetermined voltage correction values that are complementary to a FT response curve (i.e., a Bechmann curve as shown in FIG. 1) of a pretested crystal. Due to the digital nature of the compensation, a preset correction voltage is applied within a discrete, fixed temperature segment. Generally, each of these correction values is applied through a digital-to-analog conversion to a tuning circuit of the crystal so as to return the frequency of the crystal to a nominal value within that temperature segment.  
       [0014] In this solution, an integrated circuit continuously monitors the temperature in the vicinity of the crystal. The integrated circuit then applies a new voltage correction value for every five degree (5° C.) temperature segment, for example. This compensation process, however, produces a discontinuous frequency performance. That is, as the temperature fluctuates the digital-to-analog conversion output will change values in discrete steps, which then causes steps in the output frequency of the DTCXO. These abrupt frequency jumps may disrupt desired communication signals or interfere with other neighboring frequency signals. Although smaller temperature segments may be used to improve frequency stability, memory and circuit size limitations have constrained known DTCXOs to utilizing temperature segments of constant and minimum width.  
       [0015] In view of the above discussion of OCXO, TCXO, and DTCXO designs, it is apparent that there is a need in the relevant technology for a system that compensates for the FT response of a crystal oscillator while overcoming the disadvantages of the known designs. Such a compensation system should avoid the use of a compensation apparatus that applies an input to the crystal oscillator in order to modify its output and produce a compensated frequency. Such a system should also reduce the stepped frequency output associated with the DTCXO design, without increasing the complexity, size, and costs of the required circuitry. What is needed is a system that takes advantage of the superior noise characteristics of a design having an SC-cut crystal, as well as the low power consumption and compactness of a DTCXO design, while avoiding the bulkiness and large power requirements of the OCXO design.  
       SUMMARY OF THE INVENTION  
       [0016] The system and method of the present invention have several aspects, no single one of which is solely responsible for the desirable attributes of the invention. Without limiting the scope of this invention as expressed by the claims, features of the system and method will now be discussed briefly.  
       [0017] In one embodiment the invention is a system for providing a frequency correction value associated with thermally-based frequency deviations of a crystal. Such a system includes a crystal for producing an uncompensated frequency output, a thermal sensor which provides a signal reflective of the temperature of the crystal, a memory which stores data characterizing the frequency-versus-temperature responses of the crystal, and a device, responsive to the signal from the thermal sensor and the data stored in the memory, for providing a frequency correction value associated with the uncompensated frequency output of the crystal.  
       [0018] Another embodiment of the invention is an apparatus for compensating for thermally based frequency variations in an output of a piezoelectric crystal. The apparatus includes a thermal sensor which provides an output representative of the temperature of the crystal, a memory which stores a plurality of frequency correction values, wherein the plurality of correction values are associated with frequency-temperature responses of the crystal, a device responsive to the output of the thermal sensor for selecting at least one frequency correction value from the memory which correlates with said output. In such an apparatus the device, in response to the output of the crystal and the selected at least one frequency correction value, provides an output comprising a modification of the output of the crystal.  
       [0019] In yet another embodiment, the invention comprises of an apparatus for compensating for thermally based frequency variations in an uncompensated output signal associated with a piezoelectric crystal. The apparatus includes a temperature sensor which identifies a temperature representative of the temperature of the crystal, a memory which stores frequency correction values associated with the temperature of the crystal, an application which accesses the memory and retrieves at least one frequency correction value associated with the identified temperature, and at least one output which provides the uncompensated output signal and the at least one retrieved frequency correction value to a device such that the operation of the device is influenced by the uncompensated output signal and the frequency correction value.  
       [0020] In another embodiment, the invention is a method of compensating for thermally based frequency variations in an output signal associated with a piezoelectric crystal. The method includes the acts of identifying a temperature representative of the temperature of the crystal, producing a frequency correction value in response to the identified temperature, and providing the output signal and the frequency correction value to a system such that a component of the system functions in a manner which is influenced by the frequency correction value.  
       [0021] In yet another embodiment, the invention is a method of compensating for variations in output from a piezoelectric crystal which is provided to a host system. Such a method involves identifying output characteristics of the crystal in selected operating conditions, and modifying an output of the crystal, independent from influencing operation of the crystal, using at least one of the identified output characteristics to compensate for the influence of selected operating conditions on said output. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022] The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed.  
     [0023]FIG. 1 shows a FT response curve for a typical AT-cut quartz crystal.  
     [0024]FIG. 1B is a functional block diagram of an OCXO well known in the relevant technology.  
     [0025]FIG. 1C is a functional block diagram of a TCXO well known in the relevant technology.  
     [0026]FIG. 2A is a block diagram of a digital indirectly compensated crystal oscillator (DICXO) according to the present invention.  
     [0027]FIG. 2B is a functional block diagram of one embodiment of a system for frequency compensation using the DICXO of FIG. 2A.  
     [0028]FIG. 3 shows an exemplary look-up table having frequency correction values associated with temperature values in the temperature range of operation of the DICXO of FIG. 2A.  
     [0029]FIG. 4 shows a FT response curve for a SC-cut crystal used to determine the frequency correction versus temperature values of the look-up table of FIG. 3.  
     [0030]FIG. 5 shows a complete hysteresis curve and a best-fit curve for the SC-cut crystal of FIG. 4.  
     [0031]FIG. 6 shows an exemplary look-up table that may be used in conjunction with an ADC which may be included with the DICXO of FIG. 2A.  
     [0032]FIG. 7 is a flowchart illustrating a process of frequency compensation using the DICXO of FIG. 2A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0033] The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways as defined and covered by the claims. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout.  
     [0034]FIG. 2A is a block diagram showing one embodiment of a digital indirectly compensated crystal oscillator (DICXO)  210  according to the invention. The DICXO  210  is a high-precision clock device, having a non-temperature compensated crystal oscillator (XO)  220 , that provides the host system  250  with the necessary data to compensate for the FT response of the uncompensated XO  220 . The DICXO  210  thus enables the host system  250  to, for example, maintain the synchronization of the clock generators of its individual components. The individual components of the host system  250  are not shown in FIG. 2.  
     [0035] In one embodiment, the host system  250  may be, for example, a global positioning system used in airborne navigation and landing equipment; however, a DICXO  210  according to the invention may be used in, or in conjunction with, any system having digital processing capabilities and which can operate using an uncompensated reference frequency and a corresponding frequency correction value. Applications that may use the DICXO  210  include radar and ranging equipment, frequency and time measuring devices, precision time references, synthesizers, and most circuits that use numerically controlled oscillators (NCOs) well known in the relevant technology. For example, in a radar system, the DICXO  210  may be used to minimize the apparent Doppler shift caused by the temperature-induced frequency deviations of the XO  220 . The DICXO  210  enables NCOs to add in a correction term that compensates for the frequency shift of XO  220  by providing a frequency correction value. In one such application, the NCO uses the frequency correction value to accelerate the NCO cycle time by the amount required to compensate for the frequency deviation of the XO  220  from its nominal frequency. In this manner the NCO offsets the frequency output  225  of the XO  220  to restore the output frequency of the NCO to near its ideal output frequency. In yet other applications, the uncompensated reference frequency may be combined with the frequency correction value to produced an output reflecting the compensated reference frequency of the XO  220 .  
     [0036] The DICXO  210  includes a thermal sensor  230  in close proximity to the XO  220 , and a memory  240  for storing data related to the FT response of the XO  220 . In one embodiment of the invention, the XO  220 , the thermal sensor  230 , and the memory  240  may all be configured to communicate separately with the host system  250  via, for example, links  225 ,  235 , or  245  respectively. The host system  250  may comprise several components which themselves may have program applications in communication with the DICXO  210 . Additionally, the DICXO  210  may interface with, or itself be a component of, the host system  250 . As discussed further below, in one embodiment the XO  220  comprises a SC-cut crystal. Unlike prior approaches in the relevant technology, which have been unsuccessful in implementing TCXOs utilizing SC-cut crystals, the DICXO  210  in effect provides a clock device that takes advantage of the low phase noise and excellent aging characteristics of the SC-cut crystal, without the need for an oven, and enables a host system  250  to compensate for the frequency deviations of the XO  220 .  
     [0037] The XO  220  provides the host system  250  with a frequency output  225  (hereinafter “F  225 ”) representing the uncompensated frequency of the XO  220 . As previously mentioned, the XO  220  may be an SC-cut crystal. Compared to an AT-cut crystal, the SC-cut crystal has a lower noise characteristic which reduces the effects of phase noise and of spurious micro-jumps, i.e., random step frequency perturbations of very small magnitude (typically less than 20 ppb). Preferably the XO  220  is manufactured so as to minimize micro-jumps by removing contaminants and loose quartz. SC-cut crystals are generally manufactured to operate on the third overtone mode; however, the system disclosed herein can use an SC-cut crystal operating in any of its resonant frequencies. The XO  220  should be cut to minimize and balance the slope of the FT response over a specified temperature range; in this manner, the temperature sensitivity of the XO  220  remains moderate.  
     [0038] In one embodiment, the thermal sensor  230  determines the ambient temperature in the vicinity of the XO  220  and communicates it to the host system  250  via the communication link  235 . In one embodiment, the thermal sensor  230  may comprise a network of thermistors. The configuration of the thermistor network, as well as the number of thermistors included, may be based on the temperature range and frequency resolution required by the components of the host system  250 . The data stream  235  may be an analog or a digital signal. For example, in one embodiment an analog-to-digital converter device (e.g., an LTC-2400 SIGMA-DELTA ANALOG TO DIGITAL CONVERTER by LINEAR TECHNOLOGY, not shown) may be coupled to the thermal sensor  230  to convert its analog voltage output to a digital, multi-bit temperature data stream  235 .  
     [0039] As will be discussed in greater detail below with reference to FIG. 3, the memory  240  stores data related to the uncompensated FT response of the XO  220 . The memory  240 , which may be a read-only-memory (ROM), may comprise a Dallas DS1624 or DS2420, or a MICRO-CHIP 24AA65, for example. As those of ordinary skill in the relevant technology will recognize, it need not be the case that the memory  240  be incorporated within the DICXO  210 , as is shown in FIG. 2. In fact, the contents of the memory  240  may be incorporated as part of the host system  250 , for example. In one embodiment, upon powering up, the host system  250  causes the FT data stored in the memory  240  to be downloaded to a local memory store (not shown) in the host system  250 .  
     [0040]FIG. 2B is a functional block diagram of an exemplary embodiment of a system for frequency compensation using the DICXO of FIG. 2A. In this embodiment, the XO  220  provides an uncompensated frequency output Fu  225  to an application  252  of the host system  250 . The thermal sensor  230  (shown here in thermal contact with the XO  240 ) provides the application  252  with an input  235  representative of the temperature of the XO  220 . The application  252  uses the input  235  and the FT response of the XO  220  data stored in the memory  240  to derive a frequency correction value (ΔF). The application  252  may then combine the uncompensated frequency Fu  225  with the frequency correction value  245  to produce an output Fc  254  representative of the compensated frequency of the XO  220 .  
     [0041] In one embodiment, the memory  340  stores the FT response data for the XO  220  in a look-up table having a predetermined frequency correction value for each one of multiple temperature values. FIG. 3 shows a look-up table  310  such as that which may be stored in the memory  340 . T xtal    312  is the temperature (either measured or derived by interpolation) associated with a measured frequency deviation dF c    314  of the XO  220  during the calibration/production process. The frequency deviation dF c    314  is the value representing the difference between the measured, uncompensated frequency F and a specified reference frequency F r , i.e., dF c =F-F r . In this example, the frequency deviation dF c    314  is stored with a resolution of 10 μHz, wherein the tabulated values are expressed in two&#39;s compliment hexadecimal numbers. Such data points, however, might be converted to different units (e.g., Herz) appropriate for a given application. In one embodiment, F r  is the nominal frequency F 0  of the XO  220 . It should be noted, however, that the frequency deviation dF c    314  may be implemented as an absolute value (i.e., an absolute deviation regardless of sign), or even as a value representing the ratio of the frequency deviation to the reference frequency F r . The specific implementation may take several forms. What is important is that dF c    314  provide an indication of the deviation of the frequency output of the XO  220  from a reference frequency F r  as a function of the temperature of the XO  220 .  
     [0042] The host system  250  uses the data of the look-up table  310  to determine a frequency correction value by using the estimated temperature T true  and interpolating the tabulated frequency deviation dF c    314  and temperature values T xtal    312  for the XO  220 . The host system  250  derives T true  using the temperature data stream  235  provided by the thermal sensor  230 , as will be discussed further with reference to FIG. 7. In one embodiment, the look-up table  310  may be associated with a unique crystal (i.e., each DICXO  210  has a unique look-up table calculated for the actual XO  220  included in that DICXO  210 ).  
     [0043] The look-up table  310  stored in the memory  240  may be generated using the following method. In general terms, the XO  220  is subjected to at least two ambient temperature ramps to measure the FT response of the XO  220 . A hysteresis curve  510  (see FIG. 5) representing the FT response of the XO  220  is determined between a first—or low—temperature and a second—or high—temperature, and vice-versa. To obtain the data points, the ambient temperature in the vicinity of the XO  220  is varied at a constant rate from the low temperature to the high temperature, and vice-versa. The frequency deviation value dF c    314  is derived by subtracting the measured frequency of the XO  220  from its nominal frequency F o  at each of a number of points in the temperature range.  
     [0044] The following discussion provides a more detailed description of the process of deriving the values stored in the look-up table  310  of the memory  240 . FIG. 4 shows a graph of the FT response curve  402  for an exemplary XO  220  (e.g., an SC-cut crystal) as its temperature is increased. In one embodiment, the XO  220  is operated from T 1    404  (about −65° C.) to T 2    406  (about +95° C.), where the temperature of the XO  220  is increased at a rate of 2°/minute, for example. The frequency output of the XO  220  is measured every 2° C., resulting in a total of 80 data points. An additional number of temperature points, called “overshoots,” (i.e., T 0    408  and T 3    410 ) may be also measured. That is, the FT response of the XO  220  is determined from a low end overshoot temperature T 0    408  to a high end overshoot temperature T 3    410 . In the previous example, therefore, the XO  220  may be operated over a temperature range from T 0    408  (about −85° C.) to T 3    410  (about +115° C.). The XO  220  frequency output is measured every 2° C. for a total of 100 data points (i.e., 80 points from −65° C. to +95° C., plus an additional 10 overshoots at each end). The curve  402  is shown as a dotted line to emphasize frequency measurements are taken at discrete points in the operating temperature range of the XO  220 .  
     [0045] To obtain a complete hysteresis curve, the process described above is repeated in the reverse direction (i.e., from the high to the low temperature) to determine the FT response of the XO  220  at a predefined number of temperature points from T 2    406  to T 1    404  (or from the high end overshoot temperature T 3    410  to the low end overshoot temperature T 0    408 ), the temperature of the XO  220  again being decreased at a constant rate over time. In the example provided, frequency output of the XO  220  is measured every 2° C. for an additional 100 data points (i.e., 80 points from +95° C. to −65° C., plus a additional 10 overshoots at each end). FIG. 5 shows a resulting hysteresis curve  510  given by plotting all the 200 data points obtained by the above measurements—with the last 100 points described above shown by the dotted line. It should be understood that the overshoots allow for settling to complete the hysteresis curve  510 .  
     [0046] The difference between an ideal curve  520  and the hysteresis curve  510  obtained during a temperature ramp is caused by the difference in the temperature between the thermal sensor  230  and the XO  220 . This temperature differential arises from the difference in the thermal inertia properties respectively associated with the XO  220  and the thermal sensor  230 , which give rise to different time delays in the XO  220  and sensor  230  achieving the same temperature value. The difference in the time delays is defined as the thermal delay t 60 . Software algorithms, which are well known in the relevant technology, may be used to determine t 60  as well as to process the hysteresis curve  510  to create the best-fit (or “ideal”) curve  520 . Additionally, these algorithms may interpolate the raw data (i.e., the 200 data points) to fill in the look-up table  310 . These data, characterizing the FT response of the XO  220 , as well as the t 60  of the system may be stored in the memory  240 .  
     [0047] In one embodiment, the best-fit curve  520  may be the average value of the legs at each temperature point along the hysteresis curve  510 . However, depending on the needs of particular application, a more sophisticated curve fitting technique can be employed. For example, the best-fit curve can take thermal lags into account. That is, the look-up table  310  can include more than one frequency correction value for each temperature point. A first such value may represent the frequency correction that should be applied when the ambient temperature is increasing, while the second should be applied when the ambient temperature is decreasing. The host system  250  may maintain historical temperature information to determine whether the ambient temperature is increasing or decreasing and, from that information, determine which leg of the hysteresis curve  510  should be applied. The data of the best-fit curve  520  is stored in the memory  240 . Thus, during operation the memory  240  holds information as to earlier measured frequency-versus-temperature corrections.  
     [0048] It should be understood that frequency stability correction could be limited by the ability of the temperature measuring inertia of the system. Multiple ramp speeds or multiple data points, for example, can increase frequency accuracy. “Look ahead” temperature readings, along with correction components on each leg of the hysteresis curve, can also improve the frequency accuracy to the reference test data.  
     [0049] In yet another embodiment of the invention, the memory  240  may also include a second look-up table having data that the host system  250  may use to obtain a more accurate estimate of the temperature T true  of the XO  220 . FIG. 6 shows a look-up table  600  that contains analog-to-digital converter (ADC) voltage readings  604  associated with temperature values T ADC    602 . The ADC device, which has already been described with reference to FIG. 2, receives analog voltage signals from the thermal sensor  330  representing the ambient temperature in the vicinity of the XO  220 . The ADC device converts the analog voltage input to a digital data voltage reading. The host system  250  then uses the digital voltage reading and the tabulated data of look-up table  600  to obtain by interpolation a more accurate digital estimate of the temperature measured by the thermal sensor  230 . Accordingly, the non-linearity of the thermal sensor  230  is accounted for in the interpolation process. As will be further described with reference to FIG. 7, this interpolation process enhances the accuracy of determining the frequency correction value that the host system  250  must apply to the output F  325 .  
     [0050] Having described the DICXO  210 , its operation in conjunction with the host system  250  will now be described. During operation, ambient temperature fluctuations can subject the XO  220  to large frequency deviations (about 100 ppm, for example) from its nominal frequency F 0  Unlike the other methods and systems (e.g., OCXO, TCXO, and DTCXO) for compensating for the thermally-induced frequency fluctuations of a piezoelectric crystal, the compensation process of the present invention does not require that the XO  220  be subjected to some external condition (e.g., heat to elevate its temperature, or voltage to modulate its resonant frequencies). Instead, the host system  250  indirectly compensates for the frequency fluctuations of the XO  220  by using the data provided by the DICXO  210 .  
     [0051]FIG. 7 is a flowchart illustrating a process  700  of using the DICXO  210 . The process  700  starts at a state  702  after the DICXO  210  is placed in communication with the host system  250 . At a state  704 , the thermal sensor  230  periodically senses the ambient temperature in the vicinity of the XO  220 . It will be apparent to those of ordinary skill in the art that determining the temperature of the XO  220  may be accomplished in a variety of ways. For example, instead of measuring the ambient temperature in the vicinity of the XO  220 , the thermal sensor  230  may be mechanically coupled to the XO  220  to determine more directly the temperature of the XO  220 . Such mechanical coupling may be, for example, a heat-conducting adhesive connecting the XO  220  to the thermal sensor  230 . Yet another method of determining the temperature of the XO  220  is to use the XO  220  itself; such a method is well known in the relevant technology. Although the sampling rate can be any sampling rate depending on the particular application, in one embodiment the sampling rate may range from about 0.15 seconds to about 0.3 seconds, but is preferably about 0.2 seconds.  
     [0052] The process  700  now proceeds to a state  706  wherein the sensor  230  provides the host system  250  with a data stream  235  indicating the ambient temperature in the vicinity of the XO  220 . In one embodiment of the invention, the data stream  235  may be the digital values produced by an ADC converter which has received analog voltage data from the thermal sensor  230 . The host system  250  may then use the such digital data in conjunction with the look-up table  600  to derive by interpolation a digital representation T M  of the ambient temperature in the vicinity of the XO  220 .  
     [0053] The host system  250 , at a state  708 , then determines an estimate of the temperature T TRUE  of the XO  220 . T TRUE  is given by T TRUE =T M −T E , where T E =(ΔT/Δt)t 60  is the temperature error due to the temperature ramp experienced by the XO  220  during operation. The host system  250  determines in real time the values of temperature change (ΔT) over time (Δt), while the value t 60  may be retrieved from the memory  240 . For illustration purposes, a linear relationship has been used here to determine T TRUE  However, it will be apparent to a person of ordinary skill in the relevant technology that determination of T TRUE  may, for example, involve the use of historical data of the temperature of the XO  220 , wherein such data is processed using a quadratic, cubic, or any higher order, function. Additionally, in some embodiments of the invention T TRUE  may be determined using calculus techniques.  
     [0054] The process  700  continues at a state  710  where the host system  250  uses T TRUE  along with T XTAL    312  and the frequency correction dF c    314  values of the look-up table  310  to derive by interpolation a frequency correction value. Those of ordinary skill in the relevant technology will recognize that determination of dF c    314  need not be limited to a linear interpolation of the T xtal    312  and dF c    314  values tabulated in the look-up table  310 . For example, the data characterizing the FT response of the XO  220 , provided in the memory  240 , may be embodied by a mathematical expression modeling the FT response of the XO  220 . In one embodiment, an algorithm representing the mathematical model of the FT response of the XO  220  is provided in the memory  240  to the host system  250 . Such an algorithm may include, for example, the constants associated with the best-fit curve  520  as well as the form of the expression used (i.e., linear, quadratic, etc.). The host system  250  may then use the algorithm to derive a dF c    314  by providing T TRUE  as an input.  
     [0055] Having determined a frequency correction value at state  710 , the process  700  now proceeds to a state  712  wherein the host system  250  provides the frequency correction value and the uncompensated frequency F  225  of the XO  220  to its components/applications for further processing in accordance with the needs of those component/applications. In one embodiment, the host system tunes its components by applying the frequency correction value to the uncompensated frequency output F  225  of the XO  220 . The compensated frequency F c  is the frequency computed by adjusting the uncompensated frequency F by the correction dF c    314  stored in, or derived by interpolation from, the table  310  of the memory  240 . More specifically, in one embodiment of the invention the frequency deviation dF c    314  is added to the actual uncompensated DICXO  210  output frequency F  225  to yield the nominal frequency F 0  (e.g., F 0 =10 MHz). The compensated frequency F c  is valid for the ambient air temperature T M  and the uncompensated frequency F  225  measured at the sampling time.  
     [0056] In yet another embodiment of the invention, an application of the host system  250  may use the frequency error correction value dF c    314  to correct for the effects of the FT response of the XO  220 . For example, depending on the degree of accuracy desired, a calculation performed by the application may require that the corrected frequency be used instead of the nominal frequency. In some applications, offsetting software signal processing parameters, such as re-tuning numerically controlled oscillators (NCOs) will compensate for the frequency deviation of the XO  220 . In one embodiment, the host system  250  may offset an NCO to cancel the effect of the frequency error. The DICXO  210  may be used in a host system  250  that can use an uncompensated frequency  225  when the frequency deviation is provided. Those of ordinary skill in the relevant technology will recognize that there are many ways that a host system  250  may correct, offset, or cancel the frequency error arising from the FT response of an uncompensated XO  220 . With the DICXO  210  of the present invention, the correction always occurs outside the DICXO  210  and the correction methods are left to the designers of the host system  250 . Having enabled the host system  250  to synchronize the clock generators of its individual components, for example, the process  700  proceeds to a state  714  where it ends. Alternatively, in some embodiments, the process  700  may cycle through the process again, beginning at state  704 , in a continuous loop while the host system  250  is in operation.  
     [0057] In view of the above, it will be apparent that by providing a system that produces a reference frequency along with a frequency correction value, which does not utilize the expensive and cumbersome compensation components required by known compensation systems, the DICXO  210  disclosed herein overcomes long standing problems in the industry. The method and system disclosed herein avoids the use of a compensation apparatus that applies an input—other than output loading—to the crystal oscillator in order to modify its output and produce a compensated frequency. The DICXO  210  enables a host system  250  to, for example, tune the clock generator of its individual components by providing a highly accurate frequency correction value for the operating conditions (i.e., temperature and frequency) of the XO  220 , without the need for the expensive and cumbersome circuitry and components required by OCXO and TCXO designs.  
     [0058] Those skilled in the relevant technology will appreciate that numerous changes and modifications may be made to the embodiments of the invention described herein, and that such changes and modifications may be made without departing from the spirit of the invention. It is therefore intended that the appended claims cover all such equivalent variations as fall within the true spirit and scope of the invention.