Tunable capacitor

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.

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's capacitance. The following formula governs such changes:

whereC—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.

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 inFIG. 1, a tunable capacitor100includes two fixed electrodes1and2, and a sliding electrode3provided between the two fixed electrodes1and2. The tunable capacitor100is equivalent to two variable capacitors connected in series. When the sliding electrode3is in a lowest position, i.e. furthest displaced from the two fixed electrodes1and2as shown inFIG. 1, then the capacitance is minimal. However, when the sliding electrode3is in a highest position, i.e. most overlapping with the two fixed electrodes1and2, the capacitance is maximal.

Depending on the relative position of the electrodes, the tunable capacitor100provides capacitance for certain values within a particular designed range. The sliding electrode3is attached to a stepper-motor that moves the sliding electrode3between the electrodes1and2, without touching them. The gap between the first fixed electrode1and the sliding electrode3, and between the second fixed electrode2and the sliding electrode3may be air or filled with any RF dielectric, such as Teflon, or other suitable material.

The high accuracy provided by the tunable capacitor100is provided by the fixed (not movable) capacitor plates1and2, contrary to other technologies where one or two capacitor plates are movable. The sliding electrode3is 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 electrodes1or2and the sliding electrode3is decreased. Accordingly, this results in increased capacitance, according to the formula (1). Concurrently, the gap between the central sliding electrode3and the other fixed electrode2or1is 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 inFIG. 1, the total capacitance remains substantially unchanged. The fixed electrodes1and2, and the distance between the sliding electrode3, compensate each other as shown in formula (2):

1Ctot=1C1+1C2,(2)
where: Ctotis the total capacitance of the tunable capacitor,C1is the capacitance between the central electrode (3) and the side electrode (1)C2is the capacitance between the central electrode (3) and the side electrode (2)

In other implementations, a tunable capacitor200includes a fixed electrode100and two movable electrodes200, which are movable to slide relative to the fixed electrode100. As shown inFIGS. 2A-2C, the fixed electrode100is fixed by any fixing mechanism.

FIGS. 2A-2Cshow a tunable capacitor200in which movable electrodes200are connected together by a traverse300. The traverse300is preferably formed of a non-conductive material. The traverse300is connected to both movable electrodes200and preferably aligns and spaces the movable electrodes200relative to the fixed electrode100. The movable electrodes200are movable according to any number of moving mechanisms, the preferred of which are described below.

FIG. 2Ashows a tunable capacitor200that includes a threaded nut4which receives and cooperates with threaded screw5, which is turned and controlled by stepper motor6. The stepper motor6can be controlled via electrical terminals7, which can supply electrical pulses from a computer controller to the stepper motor6. The electrical pulses can include a control signal to turn the threaded screw5clock-wise or counter clock-wise, to move the movable electrodes200closer over the fixed electrode100or away from the fixed electrode100, respectively.

FIG. 2Bshows the tunable capacitor200, which includes a linear actuator9to control a push-pull rod8, to push the movable electrodes200closer over the fixed electrode100or pull the movable electrodes200away from the fixed electrode100, respectively. The linear actuator9can be controlled via electrical terminals10, which can supply electrical pulses from a computer controller to linear actuator9. The electrical pulses can include a control signal to incrementally push out or pull back the push-pull rod8.FIG. 2Cshows a tunable capacitor200in which a push-pull rod13is controlled by magnet11, around which a coil12is 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 rod13. 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 inFIGS. 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 inFIG. 2C, for example, a gap between electrodes1and2requires 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 toFIG. 2Cas an example, the maximum capacitance is achieved when the electrodes2are in the most left position overlapping the electrode1completely, 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 electrode2in squared inches;D is the gap between electrodes1and2in 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:

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:

Dissipating power of a tunable capacitor with Q=500 due to imperfect materials is:

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.