Array variable capacitor apparatus

An array variable capacitor apparatus includes a line unit including a ground line and a signal line which operates as a lower electrode; and a plurality of plates which are engaged with the line unit to generate capacitance and which operate as upper electrodes, the plurality of plates being arranged in an array pattern and having different degrees of stiffness.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Korean Patent Application No. 10-2008-0007191, filed on Jan. 23, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an array variable capacitor apparatus, and more particularly, to a variable capacitor apparatus in which variable capacitors having different degrees of stiffness are arranged in an array pattern.

2. Description of the Related Art

Radio frequency (RF) systems for processing RF signals require matching circuits or tunable filters to transceive information using wide-band or multi-band frequencies. Matching circuits or tunable filters may be implemented using a plurality of capacitor arrays.

For example, a tunable filter may employ a variable capacitor using a varactor. However, the range within which such a variable capacitor is able to be tuned to signals is limited, so it is impossible to tune to signals having variable ranges. Additionally, the quality coefficient Q of the variable capacitor may change so that performance of the filter may be reduced, and power loss can be increased.

Alternatively, a matching circuit or a tunable filter fabricated using micro-electro-mechanical systems (MEMS) technology requires an additional switch because such circuits or filters employ fixed capacitor arrays or variable capacitor arrays. Accordingly, if a matching circuit or tunable filter is fabricated using a fixed capacitor array or a variable capacitor array, the size and manufacturing costs of the matching circuit or tunable filter may increase. Additionally, an increase in parts such as switches causes insertion loss in RF signals to increase.

SUMMARY OF THE INVENTION

The present invention provides an array variable capacitor apparatus which is able to perform wide-band or multi-band tuning without using a passive element such as a switch in an radio frequency (RF) system.

According to an aspect of the present invention, there is provided an array variable capacitor apparatus including a line unit comprising a ground line and a signal line which operates as a lower electrode; and a plurality of plates which are engaged with the line unit to generate capacitance and which operate as upper electrodes, the plurality of plates being arranged in an array pattern and having different degrees of stiffness.

If a driving voltage is applied, an electrostatic force may cause the plurality of plates to move toward the line unit and to generate the capacitance, and a gap between the upper electrode and the lower electrode may change according to the stiffness.

The stiffness of each of the plurality of plates may be proportional to the size of the gap, but may be inversely proportional to the capacitance.

The stiffness of each of the plurality of plates may be adjusted according to at least one from among Young's modulus of each of the plurality of plates, the thickness of each of the plurality of plates and an area overlapping between each of the plurality of plates and the signal line.

The plurality of plates may be spaced apart from an upper portion of the line unit and arranged in an array pattern.

The apparatus may further include a plurality of support units for supporting the plurality of plates so that the plurality of plates are separated from the line unit. The plurality of support units may have different degrees of stiffness.

Each pair of the plurality of support units may be disposed at respective ends of each of the plurality of plates so as to form a bridge shape. The plurality of support units may be disposed in membrane forms at edges of each of the plurality of plates.

If the applied driving voltage reaches a pull-in voltage of at least one plate from among the plurality of plates, a gap between the at least one plate and the signal line may be reduced to the minimum level so that the capacitance may reach the maximum value.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1illustrates an array variable capacitor apparatus100according to an exemplary embodiment of the present invention, andFIG. 2schematically illustrates an equivalent circuit shown inFIG. 1. The variable capacitor apparatus100ofFIG. 1enables wide-band tuning or multi-band tuning using a plurality of variable capacitors VC#1, VC#2, . . . , VC#n, and may be employed in a matching circuit or a tunable filter of a radio frequency (RF) front end. The equivalent circuit shown inFIG. 2illustrates that first to n-th capacitors VC#1, VC#2, . . . , VC#n operate as variable capacitors.

A matching circuit refers to a circuit for impedance matching in an RF system such as an RF front end. For example, in order to design an RF system having impedance of approximately 50 ohm, if impedance of an antenna and impedance of a device connected to the antenna are adjusted to be approximately 70 ohm and 100 ohm, respectively, a matching circuit may be connected between the antenna and the device and may cause the impedance to be approximately 50 ohm. Here, the matching circuit includes an inductor and a capacitor. If the RF system is able to perform multi-band tuning, there may also be provided a plurality of matching circuits.

A tunable filter refers to a filter for selectively filtering frequencies according to the magnitude of capacitance so that only desired frequencies are obtained. The tunable filter includes an inductor and a capacitor.

Referring toFIG. 1, the variable capacitor apparatus100includes a line unit10, a plurality of support units11,21, . . . , (10m+1), and a plurality of plates12,22, . . . , (10n+2). Here, m and n are positive integers. The line unit10includes a ground line GL and a signal line SL, which operates as a lower electrode and transmits signals. For example, if the variable capacitor apparatus100is employed in the RF system, the signal line SL may transmit RF signals. The line unit10, support unit11and plate12are combined to operate as a single variable capacitor VC#1. The variable capacitors VC#1, VC#2, . . . , VC#n may be fabricated using a micro-electro-mechanical systems (MEMS) technology, which has become well-known, so detailed description thereof is omitted.

The plurality of support units11,21, . . . , (10m+1) support the plurality of plates12,22, . . . , (10n+2) so that the plurality of plates12,22, . . . , (10n+2) are spaced apart by a predetermined distance from the line unit10. The stiffness of the plurality of support units11,21, . . . , (10m+1) may be the same or different. Each pair of the plurality of support units11,21, . . . , (10m+1) may be disposed at respective ends of each of the plurality of plates12,22, . . . , (10n+2) so as to form a bridge shape.

The plurality of plates12,22, . . . , (10n+2) may operate as upper electrodes, and may be engaged with the line unit10to operate as variable capacitors VC#1, VC#2, . . . , VC#n. Each of the plurality of plates12,22, . . . , (10n+2) is separated from the line unit10in the direction of the Z axis shown inFIG. 1by the plurality of support units11,21, . . . , (10m+1). If each of the plurality of plates12,22, . . . , (10n+2) has different degrees of stiffness, the different degrees of stiffness may affect the capacitances C1, C2, . . . , Cnof the variable capacitors VC#1, VC#2, . . . , VC#n. Accordingly, the variable capacitor apparatus100is able to perform not only wide-band tuning but also multi-band tuning.

Each of the plurality of plates12,22, . . . , (10n+2) may be fabricated of metal. The stiffness of each of the plurality of plates12,22, . . . , (10n+2) may be adjusted according to the type of media, or Young's modulus and the thickness of each of the plurality of plates. Here, Young's modulus is a measure of the stiffness of the material.

If a driving voltage is applied, the plurality of plates12,22, . . . , (10n+2) may move toward the line unit10so as to generate a capacitance. The capacitance is represented by the following Equation 1.

In which, E represents the permittivity of a dielectric, namely an air gap, between each of the plurality of plates12,22, . . . , (10n+2) and the signal line SL; d represents a gap between each of the plurality of plates12,22, . . . , (10n+2) and the signal line SL; and S represents an area overlapping between each of the plurality of plates12,22, . . . , (10n+2) and the signal line SL.

Referring to Equation 1, the capacitances C1, C2, . . . , Cngenerated by each of the variable capacitors VC#1, VC#2, . . . , VC#n may be affected by the gap d between each of the plurality of plates12,22, . . . , (10n+2) and the signal line SL, and by the area S overlapping between each of the plurality of plates12,22, . . . , (10n+2) and the signal line SL. Accordingly, the stiffness of the plurality of plates12,22, . . . , (10n+2) and the stiffness of the plurality of support units11,21, . . . , (10m+1) may also affect the capacitances C1, C2, . . . , Cn. Additionally, the distance by which each of the plurality of plates12,22, . . . , (10n+2) moves toward the signal line SL may be determined according to the driving voltage and the initial capacitance of each of the variable capacitors VC#1, VC#2, . . . , VC#n.

If a negative DC driving voltage is applied to the ground line GL and if a positive DC driving voltage is applied to each of the plurality of plates12,22, . . . , (10n+2), the electrostatic force may be exerted to cause the ground line GL to pull the plates12,22, . . . , (10n+2), so that each of the plates12,22, . . . , (10n+2) may move toward the line unit10. Therefore, each of the variable capacitors VC#1, VC#2, . . . , VC#n may generate different capacitance, according to the distance of movement of the plates12,22, . . . , (10n+2) toward the line unit10. Here, the distance of movement of the plates12,22, . . . , (10n+2) toward the line unit10may be different according to the stiffness of each of the plates12,22, . . . , (10n+2).

For example, the greater the stiffness of the plates12,22, . . . , (10n+2) is, the smaller will be the distance of movement of the plates12,22, . . . , (10n+2). Accordingly, the gap d between each of the plurality of plates12,22, . . . , (10n+2) and the signal line SL may increase, and the capacitance may thus be reduced. In other words, the stiffness of the plates12,22, . . . , (10n+2) is inversely proportional to the distance of movement of the plates12,22, . . . , (10n+2), and to the capacitance. This is because the capacitance is inversely proportional to the size of the gap d between each of the plurality of plates12,22, . . . , (10n+2) and the signal line SL, namely, the gap d between the upper electrode and lower electrode.

The variable capacitors VC#1, VC#2, . . . , VC#n may have different pull-in voltages. For example, the pull-in voltages increase sequentially from the first capacitor VC#1to the n-th capacitor VC#n. A pull-in voltage refers to a voltage occurring when an upper electrode is adhered to a lower electrode with a dielectric interposed therebetween. If the applied driving voltage reaches a pull-in voltage of one selected from among the plurality of plates12,22, . . . , (10n+2), a gap between the selected plate and the signal line SL may be reduced to the minimum level so that the maximum capacitance may be generated. The pull-in voltages of the variable capacitors VC#1, VC#2, . . . , VC#n may be influenced by the stiffness of the plurality of plates12,22, . . . , (10n+2) or the stiffness of the plurality of support units11,21, . . . , (10m+1).

For example, if the driving voltage increases prior to reaching a first pull-in voltage VPI#1of the first capacitor VC#1, a gap between the first plate12and the signal line SL may be gradually reduced but capacitance C1of the first capacitor VC#1may increase. If the applied driving voltage reaches the first pull-in voltage VPI#1, an upper electrode and a lower electrode of the first capacitor VC#1may be adhered with a dielectric layer interposed therebetween, so that the gap between the first plate12and the signal line SL may be reduced to the minimum level. In other words, when the first pull-in voltage VPI#1is applied, the first plate12used as an upper electrode is adhered to the signal line SL used as a lower electrode, with an air gap formed therebetween, so the gap d between the first plate12and the signal line SL may be reduced to the minimum level, and capacitance C1may be maximized. In this situation, if pull-in voltages of other variable capacitors VC#2, . . . , VC#n are greater than the first pull-in voltage VPI#1, capacitances C2, C3, . . . , Cnmay gradually increase as the driving voltage increases.

FIG. 3is a graph showing the capacitance generated by each capacitor according to the magnitude of applied driving voltage. InFIG. 3, if the first plate12of the first capacitor VC#1does not move even when an initial driving voltage is applied, the initial value of the first capacitance C1of the first capacitor VC#1may remain unchanged. This is because the initial value of the first capacitance C1is designed to change when a driving voltage greater than a predetermined driving voltage is applied. If the driving voltage V increases and reaches the first pull-in voltage VPI#1, the first capacitance C1of the first capacitor VC#1may reach its maximum value. Additionally, since an initial value of a second capacitance C2of the second capacitor VC#2is greater than the maximum value of the first capacitance C1, the second capacitance C2of the second capacitor VC#2may start to increase after a driving voltage greater than the first pull-in voltage VPI#1is applied, and may then reach its maximum value. The maximum value of the capacitance may be generated by each capacitor according to the magnitude of the pull-in voltage.

If the same driving voltage is applied concurrently to the first to n-th capacitors VC#1, VC#2, . . . , VC#n, the gap d may change due to the different degrees of stiffness of the capacitors VC#1, VC#2, . . . , VC#n, and the capacitances C1, C2, . . . , Cngenerated by the capacitors VC#1, VC#2, . . . , VC#n may also be different from one another. Since the variable capacitors VC#1, VC#2, . . . , VC#n have different pull-in voltages, the driving voltages at which the capacitances C1, C2, . . . , Cnreach their maximum values are also different in magnitude, and the capacitances C1, C2, . . . , Cnhave different maximum values.

Therefore, when the variable capacitor apparatus100is employed in a matching circuit or a tunable filter, it is possible to perform impedance matching or multi-band frequency filtering or wide-band frequency filtering without having an additional switch. Additionally, it is possible to design the variable capacitor apparatus100to be able to perform wide-band tuning and multi-band tuning for the capacitance according to the type of RF systems including the variable capacitor apparatus100, by adjusting the stiffness and the number of capacitors, the magnitude of driving voltage and the magnitude of pull-in voltages of the capacitors VC#1, VC#2, . . . , VC#n.

For example, in a situation in which a frequency of approximately 1 GHz is filtered at a capacitance of approximately 1 picofarad and a frequency of approximately 2 GHz is filtered at a capacitance of approximately 2 picofarads, if the first capacitor VC#1and second capacitor VC#2are designed to generate the maximum capacitances of approximately 0.5 and approximately 1.5 picofarads, respectively, the first pull-in voltage VPI#1of the first capacitor VC#1and the second pull-in voltage VPI#2of the second capacitor VC#2may be applied to the variable capacitor apparatus100. Accordingly, the variable capacitor apparatus100may generate a capacitance of approximately 2 picofarads so as to filter a signal corresponding to a frequency bandwidth of 2 GHz.

FIGS. 4A and 4Billustrate an array variable capacitor apparatus200according to another exemplary embodiment of the present invention.

Referring toFIG. 4A, a plurality of support units31and41are disposed at edges of each of plates32and42.FIG. 4Aillustrates a first capacitor30and a second capacitor40for convenience of description. The plates32and42are designed to have different degrees of stiffness, and the plurality of support units31and41may also be designed to have different degrees of stiffness. The support units31and41may be fabricated as membrane forms at the edges of the plates32and42, as shown inFIG. 4A, which will affect not only the stiffness of the support units31and41but also the stiffness of the plates32and42. The form of the support units31and41is not limited to the form shown here, and the support units31and41may have various forms.

If a driving voltage is applied to a signal line SL and each of the plates32and42, a gap between the first capacitor30and the second capacitor40may be gradually reduced. For example, if the applied driving voltage reaches a pull-in voltage of the first capacitor30, a gap between the first capacitor30and the signal line SL may be reduced to the minimum level so that a first capacitance C1may be maximized. In this situation, the support units31of the first capacitor30may be inclined to cause the plate32to be attached to the signal line SL, and the support units41of the second capacitor40may be inclined to cause the plate42to be separated from the signal line SL, as shown inFIG. 4A.

Subsequently, if the applied driving voltage reaches a pull-in voltage of the second capacitor40, the plate42of the second capacitor40may be adhered to the signal line SL, as shown inFIG. 4B, so that a second capacitance C2may reach its maximum value.

Therefore, the variable capacitor apparatuses100and200according to the exemplary embodiments of the present invention are applicable to a tunable filter or a matching circuit required to implement a digital RF system or a reconfigurable RF system. Hence, the variable capacitor apparatuses100and200do not require additional switches for controlling the array configuration, so it is possible to achieve a reduction in size and in manufacturing cost. Additionally, it is possible to fabricate a high performance variable capacitor capable of performing wide-band tuning and multi band tuning by adjusting the capacitance according to changes in the stiffness and in the driving voltage.