Patent Publication Number: US-2013229747-A1

Title: Balanced voltage variable capacitor device

Description:
BACKGROUND 
     Variable capacitors may be used as capacitive tuning elements to adjust capacitance of a circuit. For example, capacitive tuning elements are used to make changes in the resonance frequencies of radio frequency (RF) resonators in nuclear magnetic resonance (NMR) spectrometers. 
     Generally, an NMR spectrometer includes a superconducting magnet for generating a static magnetic field and an NMR probe including special purpose RF coils for generating a time-varying magnetic field perpendicular to the static magnetic field, used to detect a response of a sample to the applied magnetic fields. Each RF coil and associated circuitry is able to resonate at a frequency of interest corresponding to a nucleus of interest present in the sample. In order to maximize accuracy of NMR measurements, the resonant frequency of each RF coil and associated circuitry is adjusted to equal the frequency of interest. Also, in order to maximize transfer of RF energy into the RF coils, impedance of each RF coil is matched to impedance of a transmission line and associated components used to couple RF energy into the RF coil. Thus, variable capacitors may be used to adjust the circuit resonant frequency equal the frequency of interest and/or to ensure optimal impedance matching. However, high voltages routinely applied to the variable capacitors in NMR spectrometers may lead to arcing to surrounding probe components and other undesirable effects. 
     A conventional variable capacitor generally includes one or more stator plates (electrodes) on a stator and corresponding one or more rotor plates (electrodes) on a rotor that moves in relation to the stator. The capacitance value of the variable capacitor is determined by the relative alignment of the stator plate(s) with the corresponding rotor(s), where the capacitance value generally increases with closer alignment. For example, a variable capacitor may include a rotor ring that rotates with respect to the stator, or a rotor piston that moves linearly with respect to the stator, to vary the relative alignment of the stator and rotor plates. 
     Solutions have been developed to improve voltage handling abilities of conventional variable capacitors, such as polishing the rotor piston and adding fluid to the variable capacitor. One example includes providing a unique stator shape with an extended dielectric, as described by Grossniklaus, et al., U.S. Pat. No. 7,394,642 (Jul. 1, 2008), which is hereby incorporated by reference. However, a fundamental problem remains that the variable capacitors are inherently unbalanced because they include a short or high capacitance connection at one end, such that capacitance is be varied essentially by changing the capacitance only an opposite end. 
     SUMMARY 
     In a representative embodiment, a variable capacitor device includes a stator and a rotor. The stator includes a first stator plate and a second stator plate separated from the first stator plate by an electrically insulating material. The rotor includes a first rotor plate and a second rotor plate connected by a rod, the rotor being configured to move axially for adjusting alignment of the first rotor plate relative to the first stator plate and the second rotor plate relative to the second stator plate, respectively. The first stator plate and the first rotor plate form a first variable capacitor, and the second stator plate and the second rotor plate form a second variable capacitor connected in series with the first variable capacitor. A first capacitance of the first variable capacitor and a second capacitance of the second variable capacitor are simultaneously adjustable upon the axial movement of the rotor to provide an adjustable total capacitance of the variable capacitor device. 
     In another representative embodiment, a variable capacitor device includes a housing, a stator and a rotor. The housing is formed of a dielectric material and defines a bore. The stator includes first and second stator bands formed around an outer surface of the housing, the first and second stator bands being separated by a first separating portion of the dielectric material of the housing. The rotor includes first and second rotor plates, and a connector rod connecting the first and second rotor plates, where the rotor is configured to axially move within the bore of the housing between a maximum capacitance position, in which the first rotor plate is substantially aligned with the first stator band and the second rotor plate is substantially aligned with the second stator band, and a minimum capacitance position, in which the first rotor plate is substantially aligned with the first separating portion of the dielectric material. 
     In yet another representative embodiment, a capacitor device includes a first variable capacitor, a second variable capacitor, a connector rod and an actuator rod. The first variable capacitor includes first and second plates, the first plate being stationary and the second plate being movable with respect to the first plate to provide a variable first capacitance. The second variable capacitor includes third and fourth plates, the third plate being stationary and the fourth plate being movable with respect to the third plate to provide a variable second capacitance, the second variable capacitor being connected in series with the first variable capacitor. The connector rod is configured to connect the second and third plates at a fixed distance, and the actuator rod is configured to move the second and fourth plates simultaneously. A total capacitance of the capacitor device includes a product of the first and second capacitances divided by a sum of the first and second capacitances, and a voltage applied across the capacitor device is divided between the first and second variable capacitors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The illustrative embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements. 
         FIGS. 1A and 1B  are simplified block diagrams illustrating a variable capacitor, and  FIG. 1C  is a corresponding simplified circuit diagram, according to a representative embodiment. 
         FIGS. 2A and 2B  are simplified cross-sectional diagrams illustrating a variable capacitor, according to a representative embodiment. 
         FIGS. 3A and 3B  are simplified block diagrams, and  FIG. 3C  is a corresponding simplified circuit diagram, illustrating a conventional variable capacitor. 
         FIG. 4  is a graph showing increases in withstanding voltage versus total capacitance of a variable capacitor device, according to a representative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation and not limitation, illustrative embodiments disclosing specific details are set forth in order to provide a thorough understanding of embodiments according to the present teachings. However, it will be apparent to one having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known devices and methods may be omitted so as not to obscure the description of the example embodiments. Such methods and devices are within the scope of the present teachings. 
     Generally, the various embodiments provide a variable capacitor device that includes a stator and a rotor that is movable in relation to the stator. The stator includes at least two stator plates and the rotor includes corresponding at least two rotor plates paired with the at least two stator plates, respectively. A first pair of stator and rotor plates provides a first variable capacitor and a second pair of stator and rotor plates provides a second variable capacitor, where corresponding first and second capacitances from the rotor to the stator are connected in series. The first and second capacitances vary together as the rotor moves in relation to the stator, providing a variable combined or total capacitance, where the voltage across the variable capacitor device is balanced, that is, split between the first and second variable capacitors. The variable capacitor device provides a total maximum capacitance value when the first and second pairs of stator and rotor plates are substantially aligned with one another (e.g., as close to complete overlap as the configuration allows), and a total minimum capacitance value when the first and second pairs of stator and rotor plates are substantially unaligned (e.g., as close to no overlap the configuration allows). In various embodiments, the variable capacitor device also provides any total capacitance value between when the minimum and maximum capacitance values as the rotor is moved to various positions between the minimum (substantially unaligned) and maximum (substantially aligned) positions. 
       FIGS. 1A and 1B  are simplified block diagrams illustrating a variable capacitor device, and  FIG. 1C  is a corresponding simplified circuit diagram, according to a representative embodiment. 
     Referring to  FIGS. 1A and 1B , variable capacitor device  100  includes first variable capacitor  110  and second variable capacitor  120 . The first and second variable capacitors  110  and  120  are electrically connected in series with one another, as indicated by  FIG. 1C . The first variable capacitor includes first plate  131  and second plate  141 , where the first plate  131  is stationary and the second plate  141  is movable relative to the first plate  131 . The relative positioning of the first and second plates  131  and  141  provide a variable first capacitance. For example, the first capacitance is at its maximum value when the first and second plates  131  and  141  are substantially aligned with one another (e.g., as shown in  FIG. 1A ), and the first capacitance is at its minimum value when the first and second plates  131  and  141  are substantially unaligned with one another (e.g., as shown in  FIG. 1B ). The value of the first capacitance varies along a continuum between the maximum and minimum values according to the relative position of the second plate  141  between the aligned and unaligned positions. 
     The second variable capacitor  120  similarly includes third plate  132  and fourth plate  142 , where the third plate  132  is stationary and the fourth plate  142  is movable relative to the third plate  132 . The relative positioning of the third and fourth plates  132  and  142  provide a variable second capacitance. For example, the second capacitance is at its maximum value when the third and fourth plates  132  and  142  are substantially aligned with one another (e.g., as shown in  FIG. 1A ), and the second capacitance is at its minimum value when the third and fourth plates  132  and  142  are substantially unaligned with one another (e.g., as shown in  FIG. 1B ). The value of the second capacitance varies along a continuum between maximum and minimum values according to the relative position of the fourth plate  142  between the aligned and unaligned positions. 
     The total capacitance of the variable capacitor device  100  is effectively the product of the first and second capacitances divided by the sum of the first and second capacitances. That is, the total capacitance across both the first and second variable capacitors  110  and  120  may be calculated using the formula for adding capacitors in series: 1/C 110 +1/C 120 =1/C total , where C 110  is the first capacitance, C 120  is the second capacitance, and C total  is the total capacitance. 
     Also, voltage is balanced or spread out across the variable capacitor device  100 , as opposed to being across only one of the first variable capacitor  110  or the second variable capacitor  120 . For example, when the first and second variable capacitors  110  and  120  are the same size, each of the first and second variable capacitors  110  and  120  supports about half of the total voltage across the variable capacitor device  100 . Thus, as compared to conventional capacitor devices in which only one set of variable capacitors plates (e.g., first and second plates  331  and  341  in  FIGS. 3A and 3B ) supports most of the capacitor voltage, the depicted embodiment effectively cuts the voltage in half for each of the first and second variable capacitors  110  and  120 . This enables use of smaller capacitors and significantly reduces arcing, for example. 
       FIG. 4  is a graph showing increases in withstanding voltage versus total capacitance of a variable capacitor device according to a representative embodiment. 
     Referring to  FIG. 4 , the total capacitance of the first and second variable capacitors (e.g., first and second variable capacitors  110  and  120 ) is varied from 1 ρF to 70 ρF. The withstanding voltage of the capacitor device (e.g., variable capacitor device  100 ) is measured at each of the total capacitances indicated, and compared with corresponding withstanding voltages of a conventional capacitor device (e.g., capacitor device  300 ) at the same total capacitance.  FIG. 4  shows the increase in the withstanding voltage at each total capacitance. For example, the withstanding voltage of the variable capacitor device according to a representative embodiment is doubled at 1 ρF in comparison to the conventional variable capacitor device. As the total capacitance increases from 1 ρF, the difference in withstanding voltage decreases, as indicated by the withstanding voltages corresponding to 3 ρF through 70 ρF, until reaching the maximum capacitance, at which the withstanding voltages of the capacitors are the same. This trend in behavior is optimal since the withstanding voltage of a capacitor increases with capacitance anyway. Therefore, arcing becomes less of a problem at higher capacitances. Stated differently, Z=1/jwC impedance decreases as capacitance increases. 
     Referring again to  FIGS. 1A and 1B , in the depicted embodiment, the stationary first and third plates  131  and  132  may be formed on a stator  130 , and the movable second and fourth plates  141  and  142  may be formed on a rotor  140 . Each of the stationary first and third plates  131 ,  132  and the movable second and fourth plates  141 ,  142  are formed of electrically conductive material, such as metal or metal alloy. For example, each of the stationary first and third plates  131 ,  132  and the dynamic second and fourth plates  141 ,  142  may be formed of one or more of molybdenum (Mo), tungsten (W), copper (Cu), aluminum (Al), gold (Au), or the like. 
     The stator  130  includes substrate  135 , which may be formed of an electrically insulating material, such as plastic (e.g., polytetrafluoroethylene or Teflon®), glass, ceramic, sapphire, quartz, or other dielectric material, for example. The stator  130  has a proximal end  138  and a distal end  139  in relation to an actuator (not shown), discussed below with reference to the rotor  140 . The stationary first plate  131  is formed on the surface of the substrate  135  at or near the distal end  139 , and the stationary third plate  132  is formed on the surface of the substrate  135  between the stationary first plate  131  and the proximal end  138 . In the depicted embodiment, the stationary first and third plates  131  and  132  each has a length S 1 , and are separated from one another by a first separating portion of the substrate  135  having a length S 2 . The stationary third plate  132  is separated from the proximal end  138  by a second separating portion of the substrate  135  having a length S 3 . However, the relative sizes (e.g., including lengths S 1 , S 2 , S 3 ) may be varied to provide unique benefits for any particular situation or to meet application specific requirements of various implementations. For example, the stationary first and third plates  131  and  132  may have different lengths, respectively, indicating different size first and second variable capacitors  110  and  120 , without departing from the scope of the present teachings. 
     The rotor  140  includes a connector rod  145  and an actuator rod  147 . The connector rod  145  electrically and mechanically connects the movable second and fourth plates  141  and  142  to one another. The connector rod  145  is formed of an electrically conductive material, and may be the same or different material as the second and fourth plates  141  and  142 . The actuator rod  147  is connected to the movable fourth plate  142  at an end opposite that to which the connector rod  145  is connected. The actuator rod  147  is formed of an electrically insulating material, such as ceramic, ultem, fiberglass, quartz, or other dielectric material, for example. 
     The actuator rod  147  connects the rotor  140  to an actuator (not shown). The actuator may be any device capable of moving the rotor  140  in an axial direction (indicated by “y”), such as a screw actuator, for example. The actuator may be controlled by a microcontroller or other processing device, for example, in order to adjust the rotor  140  (via the actuator rod  147 ) to the appropriate position to provide the desired total capacitance, although other means of operating the actuator rod  147  and/or controlling the actuator may be incorporated without departing from the scope of the present teachings. In various embodiments, when a processing device is used, it may be implemented by a processor, application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or combinations thereof, using software, firmware, hard-wired logic circuits, or combinations thereof. When using a processor, a memory is included for storing executable software/firmware and/or executable code that allows it to perform the various functions. The memory may be any number, type and combination of nonvolatile read only memory (ROM) and volatile random access memory (RAM). Further, the memory may include any number, type and combination of tangible computer readable storage media, such as disk drive, electrically programmable read-only memory (EPROM), electrically erasable and programmable read only memory (EEPROM), a CD, a DVD, a universal serial bus (USB) drive, and the like. 
     Similar to the stator  130 , the rotor  140  has a proximal end  148  and a distal end  149  in relation to the actuator, where the proximal end  148  moves closer to or further from the actuator (via the actuator rod  147 ). That is, operation of the actuator moves the rotor  140  in the axial direction to a position corresponding to a desired capacitance by simultaneously adjusting the respective alignments of the stationary first plate  131  and the movable second plate  141  of the first variable capacitor  110 , and the stationary third plate  132  and the movable fourth plate  142  of the second variable capacitor  120 , as discussed above. In the depicted embodiment, the rotor  140  is “floating,” meaning that the rotor  140  is not electrically conductive to the stator  130 . 
     The movable second and fourth plates  141  and  142  each has a length R 1 , and are separated from one another by the fixed length of the connector rod  145  having a length R 2 . In the depicted embodiment, the length R 1  is substantially the same as the length S 1 , and the length R 2  is substantially the same as the length R 2 . In this configuration, the first and second variable capacitors  110  and  120  are about the same size, and the corresponding first and second variable capacitances are about equal for any particular position of the rotor  140  relative to the stator  130 . However, the relative sizes (e.g., including lengths R 1 , R 2 ) may be varied to provide unique benefits for any particular situation or to meet application specific requirements of various implementations. For example, the movable second and fourth plates  141  and  142  may have different lengths, respectively, indicating different size first and second variable capacitors  110  and  120 , without departing from the scope of the present teachings. 
     As mentioned above, the total capacitance of the variable capacitor device  100  is effectively the total capacitance is the product of the first and second capacitances divided by the sum of the first and second capacitances. Also, voltage applied across the variable capacitor device  100  is divided between the first variable capacitor  110  and the second variable capacitor  120 . That is, the voltage is split, or shared by each end of the variable capacitor device  100 . Accordingly, the variable capacitor device  100  is able to handle up to about twice the voltage at low capacitance values of a conventional variable capacitor device  100  with the same total capacitance. 
     The connector rod  145  has a narrower width (or diameter) than each of the movable second and fourth plates  141  and  142 . For example, the connector rod  145  may be between about 50% and about 80% narrower than the movable second and fourth plates  141  and  142 . Accordingly, the connector rod  145  has minimal to no effect on the value of the variable second capacitance, for example, when all or a portion of the connector rod  145  is partially or substantially aligned with the stationary third plate  132  of the stator  130 . The ability of the present embodiment to simultaneously vary the first and second capacitances of the first variable capacitor  110  and the second variable capacitor  120 , respectively, enables the variable capacitor device  100  to have a higher withstanding voltage and lower minimum capacitance. 
     This differs from a conventional variable capacitor device, in which the rotor has a constant width or diameter, such that movement of the rotor relative to a stator plate does not alter capacitance of a second capacitor. For example,  FIGS. 3A and 3B  are simplified block diagrams illustrating a conventional variable capacitor device. In particular, variable capacitor device  300  includes variable capacitor  310  and fixed capacitor  320 , which are electrically connected in series with one another, as indicated by the simplified circuit diagram shown in  FIG. 3C . 
     The first variable capacitor includes first and third plates  331  and  332  on stator  330 , as well as second plate  341  on rotor  340 , where the first and third plates  331  and  332  are stationary and the second plate  341  is movable relative to the stationary first and third plates  331  and  332 . The relative positioning of the first and second plates  331  and  341  provide a variable first capacitance and the positioning of the third and second plates  332  and  341  provide a fixed second capacitance. 
     For example, the first capacitance is at its maximum value when all of the first plate  331  is aligned with the a portion of the second plate  341  (e.g., as shown in  FIG. 3A ), and the first capacitance is at its minimum value when the first plate  331  is not aligned with any portion of the second plate  341  (e.g., as shown in  FIG. 3B ). Meanwhile, the second capacitance remains essentially constant because of the uniform size of the second plate  341 , regardless of the relative positioning. Therefore, using the conventional variable capacitor device  300 , the entire range of the total variable capacitance must be covered by the variable capacitor  310 . The fixed capacitor  320  is always set to maximum capacitance and therefore always supports the minimum voltage. 
       FIGS. 2A and 2B  are simplified cross-sectional diagrams illustrating a variable capacitor device, according to a representative embodiment. 
     Referring to  FIGS. 2A and 2B , variable capacitor device  200  includes first variable capacitor  210  and second variable capacitor  220  connected in series with one another, as discussed above. However, in the depicted embodiment, the variable capacitor device  200  has a housing  205 , which may be formed of a dielectric or other electrically insulating material, examples of which are discussed above with respect to the substrate  135 . The housing  205  may have a substantially cylindrical shape, for example, having a circular or elliptical cross-section. Of course the housing  205  may incorporate various other shapes and/or cross-sections without departing from the scope of the present teachings. The housing  205  includes a circumferential outer surface  206  and defines an inner cavity or bore  207 . 
     A first stator band  231  corresponding to the first variable capacitor  210  and a second stator band  232  corresponding to the second variable capacitor  220  are formed around the circumferential outer surface  206  of the housing  205 . The first and second stator bands  231  and  232  are formed of electrically conductive material, such as metal or metal alloy. For example, each of the first and second stator bands  231  and  232  may be formed of one or more of Mo, W, Cu, Al, Au, or the like. The housing  205 , together with the first and second stator bands  231  and  232  thus form a stator  230 . The first and second stator bands  231  and  232  each have a length S 1 , and are separated from one another by a first separating portion of the material of the housing  205  indicated by length S 2 . Further, the housing  205  has a proximal end  238  and a distal end  239  determined relative to an actuator (not shown). Thus, the second stator band  232  is separated from the proximal end  238  by a second separating portion of the material of the housing  205  indicated by length S 3 . 
     A rotor  240  is positioned within the bore  207  of the housing  205 , and is configured to move axially (in the “y” direction). The rotor  240  includes first rotor plate  241 , second rotor plate  242 , connector rod  245  (in the “y” direction) and actuator rod  247 . The first and second rotor plates  241  and  242  are formed of electrically conductive material, such as metal or metal alloy. For example, each of the first and second rotor plates  241  and  242  may be formed of one or more of Mo, W, Cu, Al, Au, or the like. Each of the first and second rotor plates  241  and  242  may likewise have a substantially cylindrical shape, for example, having a circular or elliptical cross-section. Of course the first and second rotor plates  241  and  242  may incorporate various other shapes and/or cross-sections, corresponding to the shape and/or cross-section of the housing  205  and bore  207 , without departing from the scope of the present teachings. Also, each of the first and second rotor plates  241  and  242  have a length R 1 , which may be approximately the same as length S 1 , for example. Notably, the first and second rotor plates  241  and  242  may have rounded edges, as shown in  FIGS. 2A and 2B . The rounded edges on the first and second rotor plates  241  and  242  reduce charge build up on these edges, which reduces arcing. 
     In the depicted embodiment, the first rotor plate  241  and the first stator band  231  form the first variable capacitor  210 , where the position of the first rotor plate  241  relative to the first stator band  231  determines the value of a variable first capacitance. Likewise, the second rotor plate  242  and the second stator band  232  form the second variable capacitor  220 , where the position of the second rotor plate  242  relative to the second stator band  232  determines the value of a variable second capacitance. The value of the total capacitance of the variable capacitor device  200  is effectively the product of the first and second capacitances divided by the sum of the first and second capacitances, as discussed above. 
     The connector rod  245  electrically and mechanically connects the first and second rotor plates  241  and  242  to one another at a fixed distance, indicated by a length R 2 . In an embodiment, the length R 2  of the connector rod  245  may be substantially the same as the length S 2  of the first separating portion of the housing  205 . In this configuration, e.g., when the first and second stator bands have the length S 1 , the first and second rotor plates have the same length R 1 , and the length R 2  is substantially equal to the length S 2 ), the first and second variable capacitors  210  and  220  are about the same size, and the corresponding first and second capacitances are about equal for any particular position of the rotor  240  relative to the stator  230 . 
     The connector rod  245  is formed of an electrically conductive material, and may be the same or different material as the first and second rotor plates  241  and  242 . As discussed above with reference to the connector rod  145 , the connector rod  245  has a narrower width (or diameter) than each of the first and second rotor plates  241  and  242 . Accordingly, the connector rod  245  has minimal to no effect on the variable second capacitance, for example, when all or a portion of the connector rod  245  is aligned with the second stator band  232  (e.g., as shown in  FIG. 2B ). The variable capacitor device  200  is therefore able to provide a total variable capacitance that has a lower minimum value than that of conventional variable capacitor devices. In addition, the variable capacitor device  200  is able to provide multiple variable capacitors (e.g., first and second variable capacitors  210  and  220 ) connected in series, splitting the voltage on the variable capacitor device  200  among the multiple variable capacitors. 
     The actuator rod  247  is connected to the second rotor plate  242 , at an opposite end to which the connector rod  245  is connected. The actuator rod  247  is formed of an electrically insulating material, such as ceramic, ultem, fiberglass, quartz, or other dielectric material, for example. The actuator rod  247  may have a narrower thickness or diameter than the first and second rotor plates  241  and  242 , although the thickness or diameter of the actuator rod  247  has no effect on the total capacitance of the variable capacitor device  200  since it is formed of a non-conductive material. 
     The actuator rod  247  connects the rotor  240  to an actuator (not shown), which is configured to move the rotor  240  in axially (in the “y” direction) to obtain a desired capacitance by simultaneously adjusting the respective alignments of the first stator band  231  and the first rotor plate  241  of the first variable capacitor  210 , and the second stator band  232  and the second rotor plate  242  of the second variable capacitor  220  of the second variable capacitor  220 . The actuator may be any device capable of moving the rotor  240  in the axial direction, as discussed above. The rotor  240  is floating with respect to the stator  230 , as discussed above. 
     In operation, the actuator rod  247  is configured to enable movement of the rotor  240  within the bore  207  of the housing  205  via operation of the actuator. The actuator rod  247  is able to move the rotor  240  to any position between (and including) the maximum capacitance position (as shown in  FIG. 2A ) and the minimum capacitance position (as shown in  FIG. 2B ). 
     As discussed above, at the maximum capacitance position, the first stator band  231  is substantially aligned with the first rotor plate  241 , and the second stator band  232  is substantially aligned with the second rotor plate  242 . Accordingly, each of the first and second capacitances is at its maximum value, because the relative sizes of the opposing plates of the first and second variable capacitors  210  and  220  are at their maximum values. The aggregate total capacitance of the variable capacitor device  200  is likewise at its maximum value. 
     At the minimum capacitance position, the first stator band  231  is minimally aligned (substantially unaligned) with the first rotor plate  241 , and the second stator band  232  is minimally aligned (substantially unaligned) with the second rotor plate  242 . Accordingly, each of the first and second capacitances is at its minimum value, because the opposing plates of the first and second variable capacitors  210  and  220  either do not overlap, or the relative sizes of the opposing plates of the first and second variable capacitors  210  and  220  are at their minimum values. Stated differently, at the minimum capacitance position, the first rotor plate  241  is substantially aligned with the first separating portion (indicated by the length S 2 ) of the housing  205 , and the second rotor plate  242  is substantially aligned with the second separating portion (indicated by the length S 3 ) of the housing  205 . The aggregate total capacitance of the variable capacitor device  200  is likewise at its minimum value. 
     The relative sizes (including lengths) of the first and second stator bands  231  and  232  and the first and second rotor plates  241  and  242  may be varied to provide unique benefits for any particular situation or to meet application specific requirements of various implementations. Similarly, the sizes (including lengths) of the first and second separating portions of the housing  205 , indicated by the lengths S 2  and S 3 , and the size of the connector rod  245  (including diameter and length), indicated by the length R 2 , may be varied to provide unique benefits for any particular situation or to meet application specific requirements of various implementations. Of course, other sizes and/or lengths may be incorporated without departing from the scope of the present teachings. 
     As mentioned above, voltage applied across the variable capacitor device  200  is divided between the first variable capacitor  210  and the second variable capacitor  220 . That is, the voltage is split or shared by each end of the variable capacitor device  200 . Accordingly, the variable capacitor device  200  is able to handle up to about twice the voltage of a conventional variable capacitor device (e.g., variable capacitor device  300 ) using the same size capacitors. Stated differently, the variable capacitor device  200  is able to handle the same voltage as a conventional variable capacitor device using first and second variable capacitors  210  and  220  of about half the size. For example, each of the first and second variable capacitors  210  and  220  may have a corresponding capacitance range of about 2 ρF to about 140 ρF. The corresponding capacitance range of the variable capacitor device  200  is therefore between about 1 ρF and about 70 ρF, while the voltage is split between the first and second variable capacitors  210  and  220 , significantly increasing the withstanding voltage of the capacitor. 
     While specific embodiments are disclosed herein, many variations are possible, which remain within the concept and scope of the invention. Such variations would become clear after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the scope of the appended claims.