Abstract:
A single frequency control device incorporating a high frequency resonator, a low frequency resonator and a temperature sensing element, the latter thermally coupled closely to the said resonators to facilitate temperature sensing with higher resolution and accuracy. Additional benefits offered by the structure include smaller size and lower cost.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates generally to frequency control devices and, in particular, to frequency control devices that deploy high and low frequency resonators and temperature sensing elements. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    Contemporary electronic devices utilize several frequency reference components deployed to facilitate various communication functions such as cellular communications, GPS, wi-fi, Bluetooth, etc. The said frequency reference components commonly include high frequency resonators and real-time clock (RTC) resonators. 
         [0003]    In order to minimize frequency instability caused by changing ambient temperature, the high frequency resonator is often packaged together with a temperature sensing component (such as a thermistor or a diode), the latter used to sense the high frequency resonator&#39;s temperature and work out the actual resonator&#39;s frequency at that temperature. This method of computational frequency correction requires high resolution expensive analog-to-digital convertors for converting the temperature sensing voltage. When attempting to minimize the RTC frequency instability using the temperature sensing and computational correction method, one encounters the additional difficulty presented by the difference between the temperature of the RTC resonator and that of the temperature sensing element. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a single structure that incorporates three elements—a high frequency resonator, a low frequency resonator and a temperature sensing element, and wherein all three elements are closely thermally coupled so that the temperature difference between any of the three elements is further reduced. Such a structure offers the following advantages: 
         [0005]    it facilitates improved accuracy of temperature sensing and improved resolution of temperature sensing, 
         [0006]    it eliminates the need for expensive high resolution analog-to-digital convertors; 
         [0007]    it offers reduced physical size and reduced cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the accompanying drawings: 
           [0009]      FIG. 1  is a cross-sectional view of the frequency control device provided by the present invention, wherein the high frequency resonator, the low frequency resonator and the temperature sensing element are located within the same hermetic cavity. 
           [0010]      FIG. 2  is a cross-sectional view of the frequency control device provided by the present invention, wherein the high frequency resonator and the low frequency resonator are located within the same hermetic cavity, and the temperature sensing element is placed in a non-hermetic part of the structure. 
           [0011]      FIG. 3  is a cross-sectional view of the frequency control device provided by the present invention, wherein the high frequency resonator, the low frequency resonator and the temperature sensing element are housed in separate packages, the latter then assembled into a single component. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0012]    The embodiments presented herein are examples of possible implementations of the present invention. The disclosed embodiments do not limit the scope of the present invention, the said scope described in the Claims section of this disclosure. 
         [0013]    Referring to  FIG. 1 , the device presented by this invention is shown as implemented using a single multi-layer ceramic package that houses the three elements—the high frequency resonating element  1 , the low frequency resonating element  2  and the temperature sensing element  3 —all three elements positioned within the same hermetic cavity comprised of the ceramic package  4  and the lid  5 . Preferably, the resonating element  1  is an AT-cut quartz crystal, the resonating element  2  is a 32.768 kHz tuning fork crystal, and the temperature sensing element  3  is a thermistor. Alternatively, other types of high and low frequency resonators can be used instead, such as an SC-cut crystal, a surface acoustic wave (SAW) resonating element, or a MEMS resonator; also, the temperature sensing element can be alternatively implemented as a diode, a dedicated temperature sensing integrated circuit, or a resonating element whose resonant frequency is sensitive to temperature (such as, for example, a Y-cut quartz crystal). 
         [0014]    Referring to  FIG. 2 , the device shown is implemented by positioning the three elements in separate cavities of a ceramic package  4 : the high and low frequency resonating elements  1  and  2  are located in a hermetic cavity that is closed off by the lid  5 , whereas the temperature sensing element  3  is located in a non-hermetic cavity in the lower part of the package  4 . 
         [0015]    In some situations, it may be more convenient to use readily available “standard” packaged high and/or low frequency resonators as in an embodiment shown in  FIG. 3 . Therein, both the high and low frequency resonating elements ( 1  and  2  correspondingly) and enclosed in separate hermetic ceramic packages  4  and  5  closed off by lids  7  and  8 ; the temperature sensing element  3  is positioned in the cavity of the ceramic package  6  and the whole device is assembled by soldering the packaged resonating elements onto the ceramic package  6 . 
         [0016]    Other construction variants are of course possible, and a person skilled in the art will be able to develop other structures without deviating from the scope of the present invention. 
         [0017]    The close spatial proximity and the resulting thermal coupling between the three elements allow a more accurate and more efficient sensing of temperature of the two resonating elements  1  and  2 , as the said temperature sensing is done through the use of a single temperature sensing element  3  and a single measurement (or a single series of measurements pertaining to both resonating elements  1  and  2 ). The ability to use a single temperature sensing measurement for both resonating elements reduces power consumption in the application system. 
         [0018]    The close spatial proximity and the resulting thermal coupling between the three elements offer an additional advantage: the presented by this invention structure facilitates a higher resolution and cheaper temperature sensing method whereby the frequency of the low frequency resonating element (e.g., a tuning fork crystal) is used as an indication of temperature of the device. In this method, the frequency of the low frequency resonating element (e.g., a tuning fork crystal) is measured using the high frequency resonating element&#39;s signal (AT-cut crystal&#39;s frequency) as a reference frequency. In this scheme, the need for high-resolution analog-to-digital convertor required for temperature sensing using a thermistor, is eliminated since the thermistor will only be used for an approximate determination of temperature in order to determine which half of the parabolic F(T) curve of the tuning fork resonator the current temperature point “belongs” to.