Abstract:
An exemplary cavity resonator has a resonant frequency and includes a conductive body containing a cavity and a plate attached to the body enclosing the cavity. The position of a conductive tuning mechanism that protrudes into the cavity affects the tuning of the resonant frequency of the cavity resonator. A portion of the enclosed cavity is made of a shape memory alloy (SMA) material that has been trained to have a coefficient of thermal expansion that results in dimensional changes of the portion as the temperature varies so that the dimensional changes produce changes in the resonant frequency that counteract the combined change in the resonant frequency due to dimensional changes with temperature associated with the other portions of the enclosed cavity made of materials other than SMA material. This results in a stable resonant frequency versus temperature characteristic.

Description:
BACKGROUND 
       [0001]    This invention relates to a cavity resonator and more specifically relates to changes to the resonant frequency of the resonator and hence changes to the frequency characteristics of the resonator due to temperature variations. 
         [0002]    A plurality of coupled cavity resonators can be utilized to form an RF filter with a designed magnitude versus frequency characteristic. Such a filter may be formed from a metal body with separated hollow cavities defining individual resonators enclosed by a cover plate. A tuning mechanism such as metal screws with an adjustable length protruding into the hollow cavities can be utilized to fine tune each resonator to a desired frequency. The physical dimensions of the hollow cavities and the position and length of the tuning screw determines the resonant frequency of the resonator. As the materials of the resonator expand or contract due to changes in temperature, the resonant frequency of the resonator, and hence the magnitude versus frequency response of the filter, will change since the resonant frequency depends on the physical dimensions of the cavity and the position of the tuning mechanism. 
       SUMMARY 
       [0003]    It is an object of the present invention to provide a cavity resonator in which changes to its resonant frequency due to temperature variations are minimized. 
         [0004]    An exemplary cavity resonator has a resonant frequency and includes a conductive body containing a cavity and a plate attached to the body enclosing the cavity. The position of a conductive tuning mechanism that protrudes into the cavity affects the tuning of the resonant frequency of the cavity resonator. A portion of the enclosed cavity is made of a shape memory alloy (SMA) material that has been trained to have a coefficient of thermal expansion that results in dimensional changes of the portion as the temperature varies so that the dimensional changes produce changes in the resonant frequency that counteract the combined change in the resonant frequency due to dimensional changes with temperature associated with the other portions of the enclosed cavity made of materials other than SMA material. This results in a stable resonant frequency versus temperature characteristic. 
         [0005]    An exemplary method provides temperature compensation of the resonant frequency of a cavity resonator. A portion of an enclosed cavity of the cavity resonator is made of a shape memory alloy (SMA) material. The SMA material is trained to have a predetermined coefficient of thermal expansion (CTE) that results in dimensional changes of the portion as the temperature varies so that the dimensional changes produce changes in the resonant frequency that counteract the combined change in the resonant frequency due to dimensional changes with temperature associated with other portions of the enclosed cavity made of materials other than SMA material. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which: 
           [0007]      FIG. 1  shows an RF filter with 4 cavity resonators. 
           [0008]      FIG. 2  shows a cross-section of an exemplary cavity resonator in accordance with the prior art. 
           [0009]      FIG. 3  shows a cross-section of an exemplary cavity resonator in accordance with an embodiment of the present invention. 
           [0010]      FIG. 4  is a graph illustrating the exemplary training of a shape memory alloy material to exhibit a desired physical dimension versus temperature characteristic. 
           [0011]      FIG. 5  is a graph illustrating the performance of an exemplary filter with 4 resonant cavities in accordance with the prior art showing changes in performance at different temperatures. 
           [0012]      FIG. 6  is a graph illustrating the performance of an exemplary filter with 4 resonant cavities in accordance with an embodiment of the present invention having improved frequency versus temperature performance. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  shows a four stage filter  100  consisting of cavity resonators  105 ,  110 ,  115  and  120  with an input signal coupled at port  125  and a resulting filtered output signal at port  130 . Each cavity resonator is coupled to the next adjacent cavity resonator with the signal propagating left to right in filter  100 . 
         [0014]      FIG. 2  shows a cross-section of an exemplary prior art cavity resonator  200  having a body  205  dimensioned to form a cavity  210  in conjunction with a cover plate  215 . The body  205  and the cover plate  215  are made of a conductive metal such as aluminum. A mechanical tuning mechanism, e.g. tuning screws,  220  may be made of a silver alloy. Tuning screws  220  have threads that engage corresponding threads in hole  225  of the cover plate  215  permitting the length of the screw that extends below the cover plate  215  to be adjusted. A threaded sleeve  222  engages the tuning screw  220  and acts as a stop in conjunction with the top of cover  215  to secure the tuning screw  220  in a fixed position when the tuning is complete. The basic resonant frequency of the cavity resonator is determined by the volume of cavity  210 . However, the position of the end  227  of tuning screw  225  relative to the corresponding resonator rods  230  provides a tuning adjustment allowing the basic resonant frequency to be adjusted. The cavity resonator  200  as represented in  FIG. 1  is circular and is symmetrical about its axis. 
         [0015]      FIG. 3  shows a cross-section of an exemplary cavity resonator  300  in accordance with an embodiment of the present invention. Except as described below, the same elements and dimensions as previously described with regard to  FIG. 2  apply to cavity resonator  300  and hence the same reference numerals are not repeated in  FIG. 3 . The resonator rods  305 , unlike resonator rods  230  which were constructed as part of the aluminum body  205 , are formed from a shape memory alloy (SMA) material attached to the metal body. The SMA material has been trained/conditioned to provide a desired coefficient of thermal expansion (CTE) over the temperature range of interest, e.g. −25° C. to +75° C. 
         [0016]    With regard to resonator  200 , as temperature increases: 
         [0017]    (1) dimension  250  increases tending to cause the center frequency to shift higher; 
         [0018]    (2) dimension  255  increases tending to cause the center frequency to shift lower; 
         [0019]    (3) dimension  260  increases tending to cause the center frequency to shift lower. 
         [0000]    These dimensional changes are due to the coefficient of thermal expansion for the respective materials. The combined effect of these dimensional changes results in the center frequency of the passband shifting down in frequency as temperature increases. These effects are for the aluminum and silver alloy materials used for cavity resonator  200 . The same materials and dimensions as described for resonator  200  are utilized for cavity resonator  300  except for the material used for the resonator rods  305 . The cavity resonator  300 , like cavity resonator  200 , is circular and is symmetrical about its axis. 
         [0020]      FIG. 4  is a graph  400  generally illustrating the strain versus temperature characteristics of a shape memory alloy material during training steps. In accordance with the embodiment  300  of the present invention a solution for minimizing frequency versus temperature changes for the cavity resonator utilizes a two-way SMA material as part of the cavity resonator. The SMA material is trained to provide a CTE that is used to counteract/balance the naturally occurring CTE of the other materials of the cavity resonator so that resonant frequency variations with temperature are minimized. In graph  400 , curve  405  with hysteresis illustrates the strain versus temperature characteristic during a first cycle of training. The curve  405   a  represents strain as the temperature increases from −15 C to 70 C and curve  405   b  represents strain as the temperature decreases from 70 C to −15 C. The curve  410  illustrates the strain versus temperature characteristic during the 80th cycle of training representing the last training cycle having a stable characteristic as shown. The curve  410   a  represents strain as the temperature increases from −15 C to 70 C and curve  410   b  represents strain as the temperature decreases from 70 C to −15 C. The SMA material curves and training of  FIG. 4  is meant to generally illustrate SMA material characteristics and does not depict specific curves/characteristics of the resonator rod  305 . 
         [0021]    The resonator rods made of SMA material are trained to exhibit the desired geometry changes as temperature changes. The following method can be used for training a resonator rod to expand/contract a designed amount as temperature rises. 
         [0022]    1. An SMA material should be chosen so that the austensite and martensite phase change temperature will be within the maximum and minimum temperatures that the filter cavity will experience during operation. 
         [0023]    2. The SMA resonator rod is subjected to the maximum temperature in a stress-free condition. 
         [0024]    3. The SMA resonator rod is then subjected to the minimum temperature and then put under a tensile/compressive load, e.g. an amount of constant stress (megapascals), such that the measured strain equals the desired contraction/expansion of the resonator rod when the temperatures go from cold to hot. 
         [0025]    4. Continuing to maintain the constant tensile/compressive load, the SMA resonator rod is subjected to temperature cycles of minimum and maximum. This process continues until the hysteresis shape of the strain versus temperature curve stabilizes, i.e. the curve does not substantially shift with more thermal cycles. 
         [0026]    5. The SMA resonator rod has now been trained and is attached to the body of the cavity resonator and will expand or contract to the trained dimensions. 
         [0027]    The amount of tensile/compressive load to use to achieve the desired amount of dimensional change can be determined in different ways. A person can use a tensile machine that measures strain or dimensional change. The tensile load is increased until the measurement reads the desired dimensional change. Alternatively, Young&#39;s modulus of the SMA material can be looked up, where Young&#39;s modulus=stress÷strain. Dimensional change can be expressed in terms of strain. Dimensional change=strain*length of resonator. The load can be expressed in terms of stress. Load=stress*cross sectional area of resonator rod. Finally, the required load=Young&#39;s Modulus*cross sectional area of resonator rod*length of resonator÷dimensional change. 
         [0028]    A filter  100  utilizing four resonant cavities was modeled utilizing a high frequency structural simulator. Each of the four resonant cavities  200  were as shown in  FIG. 2 . The following are the dimensions of resonant cavities  200  at −25° C.: dimension  250 =0.15 inches; dimension  255 =0.12 inches; dimension  260 =0.06 inches; dimension  265 =0.04 inches. The housing, resonator rods and cover were aluminum; the tuning screw was made of coin silver alloy. 
         [0029]      FIG. 5  shows the frequency characteristic for the modeled filter  100  and how it changes at the minimum and maximum temperatures. Graph  500  has a y-axis of insertion loss of the filter in decibels and an x-axis of frequency shown in gigahertz. Curve  505  shows the loss versus frequency response at −25° C. Curve  510  shows the loss versus frequency response at +75° C. Comparing the two curves at −30 DB attenuation, curve  510  is narrower by approximately 120 MHz as compared to the curve  505 . The minimum insertion loss for curve  510  has shifted down in frequency compared with the minimum insertion loss for curve  505 . The intended center frequency of the bandpass has an overall shift downward in frequency from curve  505  to curve  510  as shown. 
         [0030]      FIG. 6  shows a graph  600  illustrating that the stabilized filter transfer characteristic versus temperature results from corresponding stabilized resonant frequency versus temperature performance of each of the four cavity resonators  300  that make up the exemplary filter. As shown, only minimal changes in performance is present over the different temperatures. A “stabilized” transfer characteristic of the filter or “stabilized” resonant frequency of the cavity resonator means that the transfer characteristic/resonant frequency of the temperature compensated filter/resonant cavity as described herein has substantially less, e.g. 50% less, variation than would occur for filters/resonant cavities without temperature compensation. The resonant cavities  300  have the same dimensions described above for resonant cavity  200  and are constructed of the same materials except for resonator rods  305  made of SMA material. This SMA material was trained to have a CTE of about 10 ppm/C as contrasted with a CTE of 23 ppm/C of resonator rods  230  made of aluminum. Curve  605  shows the frequency characteristic at −25° C.; curve  610  shows the frequency characteristic at +75° C. As demonstrated by graph  600 , there is almost no frequency shift of the passband, i.e. 6 dB crossovers. Comparing the two curves  605  and  610  at −30 dB attenuation, curve  610  is narrower by approximately 15 MHz as compared to the curve  605 . The frequency narrowing of 15 MHz at −30 dB associated with resonators  300  is an 8 to 1 improvement over the frequency narrowing of 120 MHz associated with resonators  200 . 
         [0031]    The exemplary resonator rod  305  associated with the cavity resonator  300  and the filter having the characteristics associated with graph  600  is described. The SMA material selected was nickel titanium also known as Nitinol. This material was selected since it had phase change temperatures that were within the −25° C. to +75° C. temperature range. This material has phase change temperatures: martensite finish temperature of 10 to 20 C; austensite finish temperature of 30 to 50 C. The resonator rod  305  starts with the same dimension (0.12 inches long) at −25° C. as resonator rod  230 . The resonator rod  305  would then be stretched in length to match the length of a material with a CTE of 10 ppm/C at 75° C. 
         [0032]    The required CTE of the resonator rod to achieve temperature compensation of the frequency transfer characteristic for the cavity resonator can be determined by computer simulation such as by a high frequency structural simulator. Simulations with different trial CTEs of the SMA resonant rod (and using the known CTEs of the other cavity materials) can be done to empirically determine a range of CTEs that includes the CTE that will provide the desired temperature compensation. Then trial values of CTEs with finer granularity within that range can be evaluated to determine the ideal CTE, e.g. in this embodiment about 10 ppm/C. 
         [0033]    The resonator rod is held at the stretched length of 0.12012 inches as it is repeatedly heated and cooled between −25° C. and +75° C. Since 75° C. is above the austensite finish temperature, the resonator will be trained to be at 0.12012 inches at +75° C. Since −25° C. is below the martensite finish temperature, the resonator will revert to 0.12 inches at −25° C. The resonator rod will come closer and closer to having a CTE of 10 ppm/C between −25° C. and +75° C. with more temperature cycling being performed until the SMA material stabilizes. The dimensional change (stretched length) is determined by the formula: dimensional change=CTE*original dimension (at cold temperature extreme)*change in temperature/1000000. In this resonator rod  305  example, dimensional change=10 ppm/C*0.12 inches*100° C. temperature range/1000000=0.00012 or a change from 0.12 inches to 0.12012 inches. 
         [0034]    Although exemplary implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention. For example, other parts of the cavity resonator, e.g. the tuning screw, could be made of a SMA material and trained to have an appropriate CTE to offset the other CTEs of materials of the resonator to achieve a more stable frequency characteristic versus temperature. Although the body of the cavity resonator could be constructed of an SMA material, such materials are relatively costly and hence to be cost-effective it will normally be preferable to select an element having a smaller volume to serve as a trained SMA material. 
         [0035]    The scope of the invention is defined in the following claims.