Abstract:
A trimmer capacitor is provided which includes a conductive bushing having a first terminal of the capacitor formed integrally therewith, a rotor threadably engageable with the bushing, and a dielectric portion attached at one end to the bushing and having a metallized stator surrounding the dielectric portion near the opposite end thereof. The metallized stator forms the second terminal of the capacitor, and is positioned above the bottom edge of the dielectric portion. The rotor includes transverse slots which bias the rotor in position against the bushing, to prevent undesired rotation of the rotor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a trimmer capacitor. 
         [0003]    2. Related Art 
         [0004]    Capacitors are passive electrical devices which store electrical charge. Most capacitors consist of two conductors insulated from each other by a dielectric, whereby electrical charge is stored on the conductors. Often, capacitors are used in filtration applications, such as in power supplies and in radio frequency (RF) circuits. 
         [0005]    A common type of capacitor is the “trimmer” capacitor, the capacitance of which can be varied by adjusting a portion of the capacitor (e.g., by turning a screw). Often, trimmer capacitors are used to make precise adjustments to the capacitance of a circuit, such as in microwave transceiver applications. In such applications, the trimmer capacitor can be used to adjust the resonance of an RF circuit (i.e., to “tune” the circuit) to a desired frequency. 
         [0006]    It is known to provide a trimmer capacitor having a conductive bushing, a first terminal attached to the bushing, a rotor threadably engaged to the bushing, a cylindrical dielectric portion attached at one end to the conductive bushing, and a stator attached at an opposite end of the dielectric portion, wherein the stator serves as the second terminal of the capacitor. The capacitance of such a device can be adjusted by selectively turning the rotor, which causes the rotor to advance toward or away from the stator. By varying the distance between the rotor and the stator, the capacitance of the device is adjusted. 
         [0007]    There are, however, drawbacks associated with this design, in that there are a number of components that are made separately and assembled. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention relates to a trimmer capacitor. The capacitor includes a conductive bushing having a first terminal of the capacitor formed integrally therewith, a rotor threadably engageable with the bushing, and a dielectric portion attached at one end to the bushing and having a metallized stator surrounding the dielectric portion near an opposite end of the dielectric portion. The metallized stator forms the second terminal of the capacitor, and is positioned above the bottom edge of the dielectric portion. Capacitance can be adjusted by selectively rotating the rotor, which causes the rotor to move toward or away from the stator. The rotor includes transverse slots which bias the rotor in position against the bushing, to prevent undesired rotation of the rotor. The trimmer capacitor provides manufacturing and cost advantages because it is formed from fewer components than existing trimmer capacitors. 
         [0009]    The present invention also relates to a method for manufacturing a trimmer capacitor. The method includes the steps of forming a bushing having an integral terminal and a threaded inner surface, forming a cylindrical dielectric portion, forming a metallized stator on an outer surface of the cylindrical dielectric portion, forming a rotor, attaching one end of the dielectric portion to one end of the bushing, and threading the rotor into the bushing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The foregoing features of the invention will be apparent from the following Detailed Description of the Invention, taken in connection with the accompanying drawings, in which: 
           [0011]      FIG. 1  is a side view showing the trimmer capacitor of the present invention; 
           [0012]      FIG. 2  is a top view of the trimmer capacitor shown in  FIG. 1 ; and 
           [0013]      FIGS. 3-4  are cross-sectional views showing operation of the trimmer capacitor of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present invention relates to a trimmer capacitor, as discussed in detail below in connection with  FIGS. 1-4 . 
         [0015]      FIG. 1  is a side view showing the trimmer capacitor of the present invention, indicated generally at  10 . The trimmer capacitor  10  includes a conductive bushing  12  having a cylindrical body  14  and a first terminal  16  formed integrally with and extending from the body  14 , a cylindrical dielectric portion  18 , and a stator  20  formed on an outer surface of the dielectric portion  18 . The stator  20  provides a second terminal for the capacitor  10 , and is formed so that a lower portion  22  of the dielectric portion  18  is exposed, i.e., the stator  20  is formed above the bottom edge of the dielectric portion  18 . As can be seen in the Figures, the stator  20  could comprise a cylindrical band about the dielectric portion  18 . This provides installation advantages, such that the stator  20  can be directly soldered to a printed circuit board. The stator  20  could be formed directly on a portion of the outer surface of the dielectric portion  18  using a suitable metallization process. 
         [0016]    Advantageously, by forming the first terminal  16  integrally with the bushing  12 , manufacturing steps and costs are reduced. Of course, it is noted that the first terminal  16  need not be formed integrally with the bushing  12 , and could be formed separately from the bushing  12  and subsequently attached thereto (e.g., by way of a collar extending from the first terminal  16  and press-fit over a bulge on the bushing  12 ). As will be discussed below, the bushing  12  includes internal threads for threadably receiving a rotor which can be selectively rotated with respect to the bushing  12  to adjust the capacitance of the capacitor  12 . 
         [0017]      FIG. 2  is a top view of the trimmer capacitor  10  shown in  FIG. 1 . A rotor  24  is received by, and is threadably engageable with, the body  14  of the bushing  12 , and can be selectively rotated with respect to the body  14  (as indicated by arrow A) to adjust the capacitance of the capacitor  10 . A slot  26  could be provided for accepting a screwdriver or other tool, which could be used to rotate the rotor  24 . Of course, any other type of engagement between the rotor  24  and an operating tool (e.g., Phillips-style slot, hexagonal recess, etc.) could be provided without departing from the spirit or scope of the present invention. 
         [0018]      FIGS. 3-4  are cross-sectional views showing operation of the trimmer capacitor of the present invention. As seen in  FIG. 3  (taken along the line  3 - 3  of  FIG. 2 ), the body  14  of the bushing  12  includes a threaded inner surface  34  which receives upper and lower threaded portions  28  and  32  of the rotor  24 . The upper and lower threaded portions  28  and  32  are separated by an unthreaded portion. One or more transverse slots  30  could be provided in the unthreaded portion of the rotor  24 , such that the upper threaded portion  28  is slightly offset with respect to the lower threaded portion  32 . Such an arrangement causes the threaded portions  28 ,  32  to be biased against the threaded inner surface  34 , so that unwanted rotation of the rotor  24  is reduced. The body  14  of the bushing  12  includes an annular recess  38  for receiving an upper end of the dielectric portion  18 . The dielectric portion  18  could be held in place in the annular recess  38  by way of a friction fit between the dielectric portion  18  and the annular recess  38 . It could also be held in place by an adhesive (e.g., epoxy). 
         [0019]    The rotor  24  also includes a cylindrical portion  36  which extends from the lower threaded portion  32  and is received by the dielectric portion  18 . The cylindrical portion  36  could be solid or hollow, and is in electrical communication with the bushing  12  by way of the threaded upper and lower portions  28 ,  32 . When the rotor  24  is rotated, the cylindrical portion  36  is selectively advanced toward or away from the stator  20 , as indicated by arrow B. This causes the capacitance of the device to be adjusted as desired. As shown in  FIG. 4 , the rotor  24  can be advanced (rotated) to a final position, such that the threaded lower portion  32  rests against a shoulder  19  of the dielectric portion  18  and the cylindrical portion  36  is positioned entirely within the dielectric portion  18 . The shoulder  19  prevents the rotor  24  from traveling past the threads  34  of the bushing  12 , and also prevents the cylindrical portion  36  from contacting a printed circuit board (and “shorting out” of the capacitor  10 ). 
         [0020]    The bushing  12  and rotor  24  could be formed from any suitable conductive metal, such as brass, and could be non-magnetic. The dielectric portion  18  could be formed from any suitable dielectric material, including, but not limited to, alumina (Al 2 O 3 ), zirconia, or sapphire. The stator  20  could be formed from moly-manganese, copper, tin plate, or any other suitable material. As mentioned above, the stator  20  could be formed directly on the outer surface of the dielectric portion  18  using a suitable metallization process. 
         [0021]    The trimmer capacitor  10  could be formed using the following manufacturing steps. First, the bushing  12  (including the cylindrical body  14  and integral first terminal  16 ) could be formed using a precision milling process. Then, the rotor  24  could be formed using precision milling processes. Once the dielectric portion  18  is formed by pressing and sintering, the stator  20  could be formed on a portion of the outer surface of the dielectric portion  18  using a metallization process. Once the stator  20  is formed, one end of the dielectric portion  18  is fit into the annular recess  38  of the bushing  12 . The dielectric portion  18  could be attached to the bushing  12  by way of a frictional fit, or by an adhesive (e.g., epoxy) applied to the annular recess  38  before insertion of the dielectric portion  18 . When the dielectric portion  18  is attached to the bushing  12 , the rotor  24  is threaded into the bushing  12 , forming a complete trimmer capacitor in accordance with the present invention. 
         [0022]    The trimmer capacitor of the present invention could have a wide range of operating frequencies. For example, an operating frequency range of 800 MHz to 2.1 GHz is possible, which is advantageous for usage of the trimmer capacitor in various RF applications including WiMax, cellular telephony, and global positioning system (GPS) applications. A capacity range of 0.5 to 2.5 picofarads (pF) could be provided, which corresponds roughly to 8 full rotations of the rotor of the rotor of the trimmer capacitor. A working voltage of 500 volts direct current (DC) could be provided, with a test voltage of 1,000 volts DC. The capacitor could have a quality (“Q”) rating of greater than 3,000, and an insulation resistance of greater than 10 6  megohms. An operating temperature range of −65 degrees Celsius to +125 degress Celsius is possible, and the rotor of the trimmer capacitor can be operated with torque in the range of 0.2 to 2.0 oz.-inch. Moisture resistance ratings of 10-24 hour cycles is also possible. These operational parameters could be varied without departing from the spirit or scope of the present invention. 
         [0023]    Having thus described the invention in detail, it is to be understood that the foregoing description is not intended to limit the spirit or scope thereof. What is desired to be protected is set forth in the following claims.