Dielectric resonator
The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to signal oscillators and more particularly to microwave and millimeter wave oscillators comprised of dielectric resonators. 
     2. Description of the Prior Art 
     While dielectric resonators for microwave integrated circuits are well known, a relatively new dielectric resonator having relatively good temperature stability is disclosed in a publication entitled, &#34;A Proposal of a New Dielectric Resonator Construction for MIC&#39;s&#34;, by Y. Shimoda, et al. which appeared in the IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-31, No. 7, July, 1983 at pp. 527-532. There a dielectric resonator is disclosed consisting of a cylindrical dielectric resonator element mounted on a stripline substrate by a relatively low loss dielectric mounting element. A dielectric disc having a thin metal film applied to its upper surface is affixed to the top of the resonator element. Adjustment of the operating frequency is achieved by trimming a portion of the metallized film area from the top of the dielectric disc. Such a device, however, still nevertheless exhibits resonant frequency changes as a function of temperature. 
     Accordingly, it is an object of the present invention to provide an improvement in a source of microwave and millimeter wave signals. 
     It is another object of the invention to provide an improvement in a dielectric resonator for generating microwave and millimeter wave signals. 
     It is still another object of the invention to provide a dielectric resonator having improved frequency stabilization as a function of temperature. 
     A further object of the invention is to provide for a dielectric resonator having a zero temperature coefficient. 
     And still a further object of the invention is to provide an improvement in the means for tuning the dielectric resonators. 
     SUMMARY 
     Briefly, the foregoing and other objects of the invention are provided by the method and apparatus for making a dielectric resonator substantially insensitive to temperature changes and providing an improved means for independently adjusting the resonant frequency thereof. The dielectric resonator is comprised of a generally cylindrical dielectric resonator element mounted on a stripline substrate by means of a low loss dielectric mount and having a dielectric disc rotatably attached to the top surface thereof. A zero temperature coefficient is provided by selectively choosing the thickness of the disc as a function of the temperature coefficients of the materials from which both the disc and resonator elements are fabricated. Furthermore, the top surface of the resonator element and the bottom surface of the dielectric disc include separate patterns of metallization whereby rotation of the disc provides a varying area of overlap between the patterns providing thereby a variable electromagnetic field at the resonator disc interface. Such a configuration provides a means for selectively changing resonant frequency for a particular temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the present invention is defined in the claims annexed to and forming a part of this specification, a better understanding can be had by reference to the following description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a perspective view of the preferred embodiment of the subject invention; 
     FIG. 2 is a partial perspective view of the dielectric resonator element of the embodiment shown in FIG. 1; 
     FIG. 3 is a perspective view of the underside of the dielectric disc of the embodiment shown in FIG. 1; 
     FIG. 4 is a schematic view of a first relative orientation between metallization patterns included on the top of the resonator element of FIG. 2 and the juxtaposed surface of the disc of FIG. 3; 
     FIG. 5 is a schematic view of a second relative orientation of the metallization patterns; and 
     FIG. 6 is a schematic view of a third relative orientation of the metallization patterns. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and more particularly to FIG. 1, reference numeral 10 denotes a generally cylindrical dielectric resonator element comprised of material, having a relatively high dielectric constant, such as BaTiO 3  ceramic, mounted on a stripline substrate 12 comprised of, for example, alumina ceramic, the underside of which is coated with electrical conductor material 14 for acting as a ground plane. The resonator element 10 is mounted on the upper surface of the substrate 12 by means of a generally circular mounting element 16 and which is comprised of dielectric material, having a relatively low dielectric constant, such as quartz. A strip conductor 18 as shown in FIG. 1 is located on the upper surface of the substrate adjacent the mount 16 for coupling the resonator element 10 to microwave integrated circuitry, not shown, located either elsewhere on the substrate 12 or some other piece of apparatus coupled thereto. The dielectric resonator in accordance with the subject invention also includes a dielectric disc member 20, also having a relatively low dielectric constant, located on top of the dielectric resonator element 10 and has a diameter substantially larger than the cross sectional diameter of the resonator element 10 so that it acts as a support for a pattern of metallization 22 comprised of a plurality of parallel strips of metal formed on the lower surface 24 of the disc 20 and which is intended to interact with a similar pattern of metallization 26 formed on the upper surface 28 of the resonator element 10 as shown in FIG. 2. 
     The purpose of the patterns of metallization is to adjust the resonant frequency of the dielectric resonator shown in FIG. 1 by rotating the disc 20 with respect to the dielectric resonator element 10 about a common axis 30. The percentage of overlapping metallization is adjusted causing an alteration of the electromagnetic fields in the disc 20 and thus a shift in the frequency of the composite resonator structure. The relative overlap between the two metallization patterns 22 and 26 is shown in three different orientations in FIGS. 4, 5 and 6 by changing the metallization pattern such that they are offset from the common axis 30. One position of the disc 20 can provide a complete overlap as shown in FIG. 4 to provide a substantially 50% ratio of metallized to unmetallized surface area, whereas by rotating the disc 20° by 90°, an overlap configuration as shown in FIG. 5 is provided while a full 180° rotation provides a configuration as shown in FIG. 6 where no overlap occurs but the parallel strips of the two patterns 22 and 26 are substantially aligned with one another. Accordingly, where less metallization appears at the interface between the disc 20 and the resonator element 10, the more the device resembles a dielectric resonator whereas the more metallization makes it resemble a microwave metal cavity. 
     Although the configuration as shown in FIGS. 1-3 includes a symmetrical set of metallization patterns, other variations are possible. For example, the strip patterns 22 and 26 disclosed could be in the form of a fine matrix of dots or they may be comprised of a grid of lines that can be arranged to have different periodicities at different temperatures or a Moire pattern can be provided so that the mutual area of overlap can vary over a relatively large range of temperature due to the different temperature coefficients of expansion of the resonator element 10 and disc 20. Also, when desirable, a temperature dependent angular rotation of the disc 20 with respect to the resonator element 10 can be provided, for example, by a bi-metallic strip arrangement coupled therebetween which would effect rotation as a function of temperature to provide an automatic frequency control of the resonator structure shown in FIG. 1. 
     Since a dielectric resonator structure as shown in FIG. 1 is normally temperature sensitive due to its being comprised of materials having widely different temperature coefficients of expansion, such a configuration can nevertheless be made to have a zero temperature coefficient by selectively choosing the thickness h 0  of the dielectric disc 20 as a function of the temperature coefficient of thickness expansion α h  of its constituent material, its thickness coefficient of frequency H f , and the temperature coefficient of frequency T f  of the resonator element 10 in the following manner. 
     For a disc thickness h 0 , the thickness coefficient of frequency H f  can be defined as: ##EQU1## where f 0  is the resonant frequency, and df/dh is the derivative of frequency with respect to disc thickness. 
     The temperature coefficient of frequency for the resonator element 10 can furthermore be defined by the expression: ##EQU2## where df/dT is the derivative of frequency with respect to temperature. 
     The temperature coefficient of expansion α h  for the disc 20 in the thickness direction can likewise be expressed as: ##EQU3## 
     From equation (1) it can be seen that the differential of frequency is related to that of disc thickness df(h) by: 
     
         df(h)=H.sub.f ·f.sub.0 ·dh               (4) 
    
     This means a downward frequency change for an increase in dh, since Shimoda in the above referenced publication has shown experimentally that H f  is a negative number. From equation (2) it can be seen that the differential of frequency is related to that of temperature df(T) by: 
     
         df(T)=T.sub.f ·f.sub.0 ·dT,              (5) 
    
     and which means an upward change in frequency for an increase in dT, since Shimoda&#39;s experimental data show T f  to be a positive number. In a like manner, from equation (3) the differential of thickness is related to that of temperature dh(T) by: 
     
         dh(T)=α.sub.h ·h.sub.0 ·dT,        (6) 
    
     and signifies an upward change in dh due to an increase in dT, provided α h  is positive. 
     Combining equation (6) with equation (4), one gets the expression: 
     
         df(h)=H.sub.f ·f.sub.0 ·α.sub.h ·h.sub.0 ·dT.                                             (7) 
    
     For temperature compensation, the changes in frequency from equations (5) and (7) must add to zero, i.e., 
     
         df(T)+df(h)=0.                                             (8) 
    
     Accordingly, 
     
         f.sub.0 ·dT·[H.sub.f ·α.sub.h ·h.sub.0 +T.sub.f ]=0                            (9) 
    
     from which the temperature coefficient of thermal expansion α h  of the disc thickness h that will provide temperature compensation can be expressed as: ##EQU4## 
     Thus given a resonant element 10 having a relatively high dielectric constant and knowing the temperature coefficient of thermal expansion of the relatively low dielectric constant material from which the disc 20 is comprised, the disc thickness h 0  can be determined in accordance with equation (11) to provide a zero temperature coefficient for the combined resonant structure and thereby increase frequency stabilization as a function of temperature for the constituent material from which the resonator element 10 is fabricated. Furthermore, since α h  is inversely proportional to h 0  as shown in equation (11), by making h 0  smaller α h  will be larger and vice versa. 
     Thus what has been shown and described is an oscillator for microwave or millimeter wave frequencies which is relatively insensitive to temperature changes and having a novel means of adjusting the resonator absolute frequency at a given temperature. This makes such a device particularly applicable for use with microwave integrated circuits. 
     While the present invention has been shown and described with respect to its preferred method and embodiment, it should be noted that the foregoing detailed description is made by way of illustration and not limitation. Accordingly, all modifications, alterations, and changes coming within the spirit and scope of the invention are herein meant to be included.