Abstract:
A tunable capacitor includes a first electrode and a second electrode, each being formed of a conductive material. The tunable capacitor further includes a third electrode between the first electrode and the second electrode, and a dielectric material interposed between the first electrode and the third electrode, and between the second electrode and the third electrode. The third electrode is movable relative to the first electrode and the second electrode by a stepper motor, to adjust and tune a capacitance of the tunable capacitor.

Description:
BACKGROUND 
     This document describes a tunable capacitor, and more particularly to production of electrical components for electrical circuits, specifically for precision Radio Frequency (RF) applications. 
     A capacitor is a device for storing electrical energy. The amount of stored energy is defined as a capacitance of the capacitor, which is measured in units of Farads. Some capacitors can be tuned, i.e. having a variable capacitance, but adjustable to a particular capacitance. Such tunable capacitors are sometimes referred as variable capacitors, trimmer-capacitors, or simply “trimmers”. 
     Trimmers come in a variety of sizes and levels of precision. The capacitance of trimmers can be adjusted with a small screwdriver, in which several turns of an adjustment screw can reach a desired end value, allowing for some degree of accuracy. Conventional trimmers include two electrically conductive electrodes separated by a dielectric material, and the distance between the electrodes and/or dielectric material affects the capacitance. To tune a trimmer, the distance between the electrodes or overlapping area of the electrodes is changed, and results in changing the capacitor&#39;s capacitance. The following formula governs such changes: 
     
       
         
           
             
               
                 
                   
                     C 
                     = 
                     
                       
                         ɛ 
                         · 
                         S 
                       
                       d 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where
         C—capacitance of the trimmer,   ∈—dielectric constant of dielectric,   S—overlapping area,   d—distance between the electrodes       

     Conventional trimmers, however, are not very accurate, and have limited range of capacitance value. Further, they do not allow automatic digital control of the capacitance value with high accuracy, as is required for such applications as tunable RF filters. 
     SUMMARY 
     This document presents a tunable capacitor that overcomes the limitations of conventional tunable capacitors and trimmers. The tunable capacitor of the present disclosure is highly accurate, provides a large range of capacitance value, and allows for automatic digital control of the capacitance value. Further, the tunable capacitor described herein has high power handling capability. 
     In some implementations, a tunable capacitor is embodied as a mechanically tunable trimmer, in which a capacitance of the tunable capacitor can be adjusted or tuned by means of an external control. The external control can be a mechanical driver powered by a stepper motor. In preferred instances, the stepper motor motion is controlled digitally from a computer in communication with the stepper motor. 
     In one aspect, a tunable capacitor is disclosed. The tunable capacitor includes a first electrode and a second electrode, wherein each of the first and second electrodes are formed of a conductive material. The tunable capacitor further includes a third electrode between the first electrode and the second electrode. The tunable capacitor further includes a dielectric material interposed between the first electrode and the third electrode, and between the second electrode and the third electrode. The third electrode is movable relative to the first electrode and the second electrode by a stepper motor, to adjust and tune a capacitance of the tunable capacitor. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects will now be described in detail with reference to the following drawings. 
         FIG. 1  illustrates a tunable capacitor in accordance with implementations of the present disclosure. 
         FIGS. 2A-2C  illustrate various mechanisms for implementing a tunable capacitor. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     This document describes a tunable capacitor, and more particularly a mechanically tunable capacitor having high accuracy in the designed range. Further, the tunable capacitor described herein provides a large range of capacitance value and allows for automatic digital control of the capacitance value. 
     In accordance with some implementations, as shown in  FIG. 1 , a tunable capacitor  100  includes two fixed electrodes  1  and  2 , and a sliding electrode  3  provided between the two fixed electrodes  1  and  2 . The tunable capacitor  100  is equivalent to two variable capacitors connected in series. When the sliding electrode  3  is in a lowest position, i.e. furthest displaced from the two fixed electrodes  1  and  2  as shown in  FIG. 1 , then the capacitance is minimal. However, when the sliding electrode  3  is in a highest position, i.e. most overlapping with the two fixed electrodes  1  and  2 , the capacitance is maximal. 
     Depending on the relative position of the electrodes, the tunable capacitor  100  provides capacitance for certain values within a particular designed range. The sliding electrode  3  is attached to a stepper-motor that moves the sliding electrode  3  between the electrodes  1  and  2 , without touching them. The gap between the first fixed electrode  1  and the sliding electrode  3 , and between the second fixed electrode  2  and the sliding electrode  3  may be air or filled with any RF dielectric, such as Teflon, or other suitable material. 
     The high accuracy provided by the tunable capacitor  100  is provided by the fixed (not movable) capacitor plates  1  and  2 , contrary to other technologies where one or two capacitor plates are movable. The sliding electrode  3  is movable, and is not electrically connected to any circuit (or ground); it is an electrically isolated electrode, which is easier to move without compromising electrical performance. 
     Further, the gaps between the electrodes need not be kept constant for higher accuracy, as is the case for some conventional capacitors. Assuming that the central electrode deviates from its central position to one side, the gap between one of the fixed electrodes  1  or  2  and the sliding electrode  3  is decreased. Accordingly, this results in increased capacitance, according to the formula (1). Concurrently, the gap between the central sliding electrode  3  and the other fixed electrode  2  or  1  is increased, which results in decreased capacitance, according to formula (1). Thus, due to the series connection of the two capacitive arrangements, created by the two gaps as shown in  FIG. 1 , the total capacitance remains substantially unchanged. The fixed electrodes  1  and  2 , and the distance between the sliding electrode  3 , compensate each other as shown in formula (2): 
                       1     C   tot       =       1     C   1       +     1     C   2           ,           (   2   )               
where: C tot  is the total capacitance of the tunable capacitor,
         C 1  is the capacitance between the central electrode ( 3 ) and the side electrode ( 1 )   C 2  is the capacitance between the central electrode ( 3 ) and the side electrode ( 2 )       

     In other implementations, a tunable capacitor  200  includes a fixed electrode  100  and two movable electrodes  200 , which are movable to slide relative to the fixed electrode  100 . As shown in  FIGS. 2A-2C , the fixed electrode  100  is fixed by any fixing mechanism. 
       FIGS. 2A-2C  show a tunable capacitor  200  in which movable electrodes  200  are connected together by a traverse  300 . The traverse  300  is preferably formed of a non-conductive material. The traverse  300  is connected to both movable electrodes  200  and preferably aligns and spaces the movable electrodes  200  relative to the fixed electrode  100 . The movable electrodes  200  are movable according to any number of moving mechanisms, the preferred of which are described below. 
       FIG. 2A  shows a tunable capacitor  200  that includes a threaded nut  4  which receives and cooperates with threaded screw  5 , which is turned and controlled by stepper motor  6 . The stepper motor  6  can be controlled via electrical terminals  7 , which can supply electrical pulses from a computer controller to the stepper motor  6 . The electrical pulses can include a control signal to turn the threaded screw  5  clock-wise or counter clock-wise, to move the movable electrodes  200  closer over the fixed electrode  100  or away from the fixed electrode  100 , respectively. 
       FIG. 2B  shows the tunable capacitor  200 , which includes a linear actuator  9  to control a push-pull rod  8 , to push the movable electrodes  200  closer over the fixed electrode  100  or pull the movable electrodes  200  away from the fixed electrode  100 , respectively. The linear actuator  9  can be controlled via electrical terminals  10 , which can supply electrical pulses from a computer controller to linear actuator  9 . The electrical pulses can include a control signal to incrementally push out or pull back the push-pull rod  8 .  FIG. 2C  shows a tunable capacitor  200  in which a push-pull rod  13  is controlled by magnet  11 , around which a coil  12  is wound. Direct current signals form an external source, such as a computer or other logical controller, controls a magnetic force exerted on the push-pull rod  13 . These implementations provide accuracy of precision motion and do not require a high control voltage like many conventional trimmers. 
     In preferred implementations, control voltage terminals and the RF signal terminals are separated, which does not require a DC block circuit. As a result, the quality of the tunable capacitor is much higher than conventional designs. In addition, the tunable capacitor described herein, especially as shown in  FIGS. 2A and 2B , has no springs and is insensitive to vibration. The capacitive value does not depend on a position of the tunable components, and therefore any error is eliminated. 
     The tunable capacitor can handle high power, and has a dielectric strength to be able to withstand 1000 volts or more. In preferred implementations, the tunable capacitor uses an aluminum oxide, or “alumina” dielectric having a dielectric constant of approximately 9.5. Other dielectric materials can be suitably used, such as polytetrafluoroethylene, otherwise known as Teflon®, for example. Referring back to the exemplary implementation shown in  FIG. 2C , for example, a gap between electrodes  1  and  2  requires a dielectric thickness of approximately 0.010″, and the dimensions of electrodes are approximately 0.400″×0.200″. Accordingly, the overall dimensions of the capacitor is 0.400″×0.400″×0.100″. Of course, these dimensions are exemplary, and actual dimensions could vary by up to 10% or more from those disclosed. 
     The tunable capacitor described herein that uses alumina dielectric can withstand up to 1055V or more, while a tunable capacitor using a Teflon dielectric can withstand up to 4700V or more. Accordingly, the tunable capacitor described herein can withstand high power as well. 
     Referring to  FIG. 2C  as an example, the maximum capacitance is achieved when the electrodes  2  are in the most left position overlapping the electrode  1  completely, and can be calculated as follows: 
                       C   ⁡     [   pF   ]       =       0.2249   *   ε   *     S   ⁡     [     sq   .           ⁢   in   .     ]           2   *     D   ⁡     [   in   ]             ,           (   2   )               
Where: C is the capacitance in Pico farads;
         ∈ is dielectric constant of the capacitor dielectric, (i.e. 9.5 for Alumina, 2.1 for Teflon);   S is the area of the electrode  2  in squared inches;   D is the gap between electrodes  1  and  2  in inches.       

     This formula (2) results in maximum capacitance value of approximately 8.5 pF, which is sufficient for an RF application. The minimum value is close to 0 pF. 
     The break-down voltage for the tunable capacitor in which alumina is used for the dielectric can be given as: 
     
       
         
           
             
               V 
               = 
               
                 
                   
                     13400 
                     ⁡ 
                     
                       [ 
                       
                         V 
                         mm 
                       
                       ] 
                     
                   
                   * 
                   2 
                   * 
                   
                     0.010 
                     ″ 
                   
                   * 
                   3.937 
                 
                 = 
                 
                   1055 
                   ⁡ 
                   
                     [ 
                     V 
                     ] 
                   
                 
               
             
             , 
           
         
       
     
     For a Teflon dielectric, the break-down voltage is even higher, around 4700[V]. The reactive power stored in the capacitor, then, can be calculated using the following formula: 
     
       
         
           
             
               
                 
                   
                     P 
                     = 
                     
                       
                         
                           C 
                           · 
                           · 
                           
                             V 
                             2 
                           
                         
                         2 
                       
                       = 
                       
                         
                           
                             8.5 
                             · 
                             
                               10 
                               
                                 - 
                                 12 
                               
                             
                             · 
                             
                               1055 
                               2 
                             
                           
                           2 
                         
                         = 
                         
                           4.7 
                           ⁡ 
                           
                             [ 
                             μW 
                             ] 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Dissipating power of a tunable capacitor with Q=500 due to imperfect materials is: 
     
       
         
           
             
               P 
               dis 
             
             = 
             
               
                 P 
                 Q 
               
               = 
               
                 
                   4.7 
                   500 
                 
                 = 
                 
                   9.5 
                   ⁡ 
                   
                     [ 
                     nW 
                     ] 
                   
                 
               
             
           
         
       
     
     This is a very small power, and cannot damage the tunable capacitor. However, as described above, power is not the damaging factor; the voltage is. The tunable capacitor can withstand 1055V with alumina and 4700V with Teflon dielectric. Accordingly, the tunable capacitor can withstand high power as well, the threshold of which can be estimated only for a particular application in which the capacitor is used. 
     Although a few embodiments have been described in detail above, other modifications are possible. Other embodiments may be within the scope of the following claims.